Operation Manual
Model 2105
Wide-Range Digital Power Analyzer
VA L HA L LA
SCIENTIFIC
Need Help? Call 800-548-9806.
www.valhallascientific.com
©2015 Valhalla Scientific, Inc. All Rights Reserved.
Updated March 2015
VALHALLA SCIENTIFIC, INC. CERTIFICATION
Valhalla Scientific, Inc. certifies that this instrument was thoroughly tested and inspected and
found to meet published specifications when shipped from the factory. Valhalla Scientific, Inc.
further certifies that its calibration measurements are traceable to the Nation Institute of
Standards and Technology to the extent allowed by NIST’s calibration facility. Due to continuing
product refinement and due to possible parts manufacturer changes, Valhalla Scientific, Inc.
reserves the rights to change any or all specifications without notice.
VALHALLA SCIENTIFIC, INC. WARRANTY STATEMENT
The warranty period for this instrument is stated on your invoice and packing list. Please refer
to these to determine appropriate warranty dates.
We will repair the instrument during the warranty period provided it is returned to Valhalla
Scientific, Inc. freight prepaid. No other warranty is expressed or implied.
Valhalla Scientific, Inc. is not liable for consequential damages.
Permission and a Return Material Authorization number (RMA) must be obtained directly from
the factory for warranty repairs. No liability will be accepted if returned without such
permission.
Due to continuing product refinement and due to possible parts manufacturer change, Valhalla
Scientific reserves the right to change any or all specifications without notice.
2105 Wide Range Power Analyzer
Operation & Maintenance Manual
Edition 2
Copyright © 2015-2016 Valhalla Scientific, Inc. All Rights Reserved
2
DOCUMENTATION HISTORY
This icon accompanies text and/or other international symbols dealing with hazards
This icon accompanies text and/or other international symbols dealing with potential
are executed incorrectly.
Paragraphs marked by this symbol indicate that a potential hazard to your personal
personnel.
Indicates the field wiring terminal that must be connected to ground before
All Editions and Updates of this manual and their creation date are listed below. The first
Edition of the manual is 1. The Edition Number increments by 1 whenever the manual is
revised. Updates, which are issued between Editions, contain replacement pages to correct or
add additional information to the current Edition of the manual. Whenever a new Edition is
created, it will contain all of the Update information for the previous Edition. Each new Edition
or Update also includes a revised copy of this documentation history page.
Edition 1 ..................................................................................................................... October, 2007
Edition 2 ........................................................................................................................ March, 2015
SAFETY SYMBOLS
to personnel. When present, it indicates that a potential hazard to your personal
safety exists if information stated within the "WARNING" paragraph is not adhered
to or procedures are executed incorrectly.
damage to equipment. When present, it indicates that there is a potential danger of
equipment damage, software program failure or that a loss of data may occur if
information stated within the "CAUTION" paragraph is not adhered to or procedures
safety exists from a high voltage source. In this document, the "WARNING" icon will
accompany this symbol. To reduce risk of electrical shock, all devices employ a threewire electrical cable and plug to connect the equipment to earth ground. Ensure that
the wall outlet receptacle is properly wired and earth grounded.
DO NOT use a three-to-two wire plug adapter.
DO NOT use a two wire extension cord or a two wire multiple-outlet power strip.
Disconnect power to the system before performing maintenance.
DO NOT remove any panels; panels should be removed only by qualified service
operation the equipment --- protects against electrical shock in case of fault.
3
WARNINGS
The following general safety precautions must be observed during all phases of operation,
service, and repair of this product. Failure to comply with these precautions or with specific
warnings elsewhere in this manual violates safety standards of design, manufacture, and
intended use of the product. Valhalla Scientific assumes no liability for the customer’s failure to
comply with these requirements
Ground the equipment: For Safety class 1 equipment (equipment having a protective earth terminal),
an interrupted safety earth ground must be provided from the main power source to the product input
wiring terminals or supplied power cable.
DO NOT operate the product in an explosive atmosphere or in presence of flammable gases or fumes.
For continued protection, replace the line fuse(s) only with fuse(s) of the same voltage and current
rating and type. DO NOT use repaired fuses or short-circuited fuse holders.
Keep away from live circuits: Operating personnel must not remove equipment covers or shields.
Procedures involving the removal of covers or shields are for use service trained personnel only. Under
certain conditions, dangerous voltage may exist even with the equipment switched off. To avoid
dangerous electrical shock, DO NOT perform procedures involving cover or shield removal unless you
are qualified to do so.
DO NOT operate damaged equipment: Whenever it is possible that the safety protection features built
into this product have been impaired, either through physical damage, excessive moisture, or any
reason, REMOVE POWER and do not use the product until safe operation can be verified by servicetrained personnel. If necessary, return the product to Valhalla Scientific for service and repair to ensure
that safety features are maintained.
DO NOT service or adjust alone: Do not attempt internal service or adjustment unless another person,
capable of rendering first aid and resuscitation, is present.
DO NOT substitute parts or modify equipment: Because of the danger of introducing additional
hazards, do not install substitute parts or perform any unauthorized modification to the product. Return
the product to Valhalla Scientific for service and repair to ensure that safety features are maintained.
Measuring high voltage is always hazardous: ALL multimeters input terminals (both front and rear)
must be considered hazardous whenever inputs greater than 42V (dc or peak) are connected to ANY
input terminal.
Permanent wiring of hazardous voltage or sources capable of delivering greater than 150VA should be
labeled, fused, or in some other way protected against accidental bridging or equipment failure.
DO NOT leave measurement terminals energized when not in use.
4
Table of Contents
Chapter 1 General Information ................................................................................................. 9
1.1 Introduction...................................................................................................................... 9
1.2 Manual Description .......................................................................................................... 9
1.3 Instrument Identification ................................................................................................. 9
Chapter 2 Unpacking and Installing ......................................................................................... 10
2.1 Introduction.................................................................................................................... 10
2.2 Inspection ....................................................................................................................... 10
2.3 Line Voltage/Fuse Selection ........................................................................................... 10
2.4 Bench Use ....................................................................................................................... 10
2.5 Rack Mounting ............................................................................................................... 11
2.6 Safety Precautions .......................................................................................................... 11
Chapter 3 Specifications .......................................................................................................... 12
3.1 Range and Resolution Table ........................................................................................... 12
3.2 Accuracies ....................................................................................................................... 12
3.3 Operating Specifications ................................................................................................ 13
3.4 Environmental and Physical Specifications .................................................................... 13
Chapter 4 Optional Equipment ................................................................................................ 14
4.1 Optional Accessories ...................................................................................................... 14
Chapter 5 Operations .............................................................................................................. 15
5.1 General ........................................................................................................................... 15
5.2 Front Panel Controls ....................................................................................................... 15
5.2.1 Power Switch .......................................................................................................... 15
5.2.2 V/I Key and Amps/Volts Display Indicator .............................................................. 15
5
5.2.3 Current Range Keys ................................................................................................. 15
5.2.4 Voltage Range Keys ................................................................................................. 15
5.2.5 Peak Amp Overload Indicator ................................................................................. 15
5.2.6 Peak Volt Overload Indicator .................................................................................. 16
5.2.7 Amps-Volts Display ................................................................................................. 16
5.2.8 Watts – True Power Display .................................................................................... 16
5.3 Rear Panel Controls ........................................................................................................ 17
5.4 Making Connections ....................................................................................................... 17
5.4.1 Using Option X-21 ................................................................................................... 18
5.4.2 Using CT’s and PT’s ................................................................................................. 18
5.4.2.1 Current Transformers .......................................................................................... 18
5.4.2.2 Potential Transformers ....................................................................................... 19
5.4.2.3 Using Both CT’s and PT’s ..................................................................................... 20
5.5 Manipulating the Data ................................................................................................... 21
5.5.1 VA, VAR, PF and ϕ ................................................................................................... 21
5.5.2 PF, Leading or Lagging? ........................................................................................... 22
5.5.3 Accuracies ............................................................................................................... 22
Chapter 6 Routine Maintenance ............................................................................................. 23
6.1 General ........................................................................................................................... 23
6.2 Periodic Maintenance .................................................................................................... 23
6.2.1 Cleaning ................................................................................................................... 23
6.2.2 Tightening ............................................................................................................... 23
6.3 Calibration ...................................................................................................................... 24
6.3.1 Equipment Required ............................................................................................... 24
6
6.3.2 ADC Reference Adjustments ................................................................................... 24
6.3.3 Calibration Connections .......................................................................................... 25
6.3.4 Offset Adjustment ................................................................................................... 25
6.3.5 -5 Volt Reference Adjustments ............................................................................... 25
6.3.6 Display Adjustments ............................................................................................... 25
6.3.7 Linearity Adjustments ............................................................................................. 26
6.3.8 20A Range Adjustments .......................................................................................... 26
6.3.9 2A Range Adjustments ............................................................................................ 26
6.3.10 0.2A Range Adjustments ......................................................................................... 26
6.3.11 Watts Zero and Offset Adjustments ....................................................................... 26
6.3.12 Full Scale Watts Adjustments ................................................................................. 26
6.3.13 AC Adjustments and Checks ................................................................................... 27
Chapter 7 Theory of Operations .............................................................................................. 28
7.1 Troubleshooting ............................................................................................................. 28
7.1.1 Component Replacement ....................................................................................... 28
7.1.2 Finding the Faulty Component ............................................................................... 29
7.2 Functional Descriptions .................................................................................................. 29
7.3 Detailed Circuit Descriptions .......................................................................................... 30
7.3.1 Power Supply .......................................................................................................... 31
7.3.2 Voltage Amplifier and Scaling ................................................................................. 31
7.3.3 Current Amplifiers and Scaling ............................................................................... 32
7.3.4 Peak Detector/Indicators ........................................................................................ 34
7.3.5 Analog to Digital Converters ................................................................................... 34
7.3.5.1 Volts/Amps Display ............................................................................................. 34
7
7.3.5.2 Watts Display....................................................................................................... 35
7.3.6 Decimal Point Positioning ....................................................................................... 35
7.3.7 Analog Multiplier .................................................................................................... 35
7.3.8 Range Selection ....................................................................................................... 35
Chapter 8 Addendums ............................................................................................................. 36
Chapter 9 Parts List .................................................................................................................. 37
Main Board Assembly ............................................................................................................... 38
Display Board Assembly ............................................................................................................ 43
Switch Board Assembly ............................................................................................................. 44
Chassis Assembly ...................................................................................................................... 45
Chapter 10 Schematics .............................................................................................................. 46
8
Chapter 1 General Information
1.1 Introduction
This manual has information to perform Inspection, Installation, Measurement Operations, and
Troubleshooting for Valhalla Scientific Model 2105. Also included in this manual are:
Specifications, Parts Lists and Schematics.
1.2 Manual Description
This Manual is separated into the 9 following Chapters.
Chapter 1 – General Information
Chapter 1 contains a brief description of the
instrument and other general information.
Chapter 2–Unpacking and Installing
Chapter 2 contains information for unpacking
and inspection the equipment. Also contains
power requirements and installation procedure.
Chapter 3 - Specifications
Chapter 3 contains all specifications, including
Ranges, Resolutions, Accuracy, General and
Physical Specifications for Valhalla Model 2105.
Chapter 4 – Options and Accessories
Chapter 4 contains information on all available
options and test leads for the Valhalla Model
2105.
Chapter 5 – Operations
Chapter 5 describes all operations including a
description of the front and rear panel,
Measurements and Connections.
Chapter 6 – Routine Maintenance
Chapter 6 contains maintenance information,
such as calibration procedure.
Chapter 7 – Theory of Operation
Chapter 7 describes the theory of operation of
all circuits and sub circuits used on Valhalla
Model 2105.
Chapter 8 – Addendums
Chapter 8 lists updates and addendums for this
manual.
Chapter 9 – Parts List
Chapter 9 contains all parts used in the Main
Board, Display Board, and Chassis of the
Valhalla Model 2105. The parts are listed with a
Reference Designator, Valhalla Part Number,
Description, and Manufacturers Part Number.
Chapter 10 – Schematics
Chapter 10 contains all schematics and
drawings for Valhalla Model 2105.
1.3 Instrument Identification
Valhalla Scientific instruments are identified by a two part serial number. The Serial Tag is
located on the rear or bottom of the instrument. The number is in a form of 00-0000. The first
two digits, called the serial number prefix, indicate the model. It changes only when a change is
made to the instrument. The last 3 or 4 digits, called the serial number suffix, are unique for
each individual unit. Be sure to include the entire serial number, both prefix and suffix, in any
correspondence about your instrument.
The serial number can also be found on the Main Board, Calibration Tag and Certificate of
Calibration.
9
Chapter 2 Unpacking and Installing
2.1 Introduction
Valhalla Scientific Model 2105 is accurate, reliable low-cost power measurement devices
designed to aid engineering, production test, and quality assurance departments in
determination of product power consumption from DC and AC power sources. The instruments
feature dual independent digital displays. The left display provides a continuous indication of
true power in watts. The right display is switch-selectable between amperes (true RMS) or volts
(true RMS).
Model 2105 provides a fast and convenient method of determining product efficiency, power
factor, and true RMS current draw. Phase angle relationships may be calculated through
manipulation of the displayed quantities.
2.2 Inspection
If the shipping carton is damaged, request that the carrier's agent be present when the unit is
unpacked. If the instrument appears damaged, the carrier's agent should authorize repairs
before the unit is returned to the factory. Even if the instrument appears undamaged, it may
have suffered internal damage in transit that may not be evident until the unit is operated or
tested to verify conformance with its specifications. If the unit fails to operate, notify the
carrier's agent and the nearest Valhalla Sales Office. Retain the shipping carton for the carrier's
inspection. DO NOT return equipment to Valhalla Scientific or any of its sales offices prior to
obtaining authorization to do so.
2.3 Line Voltage/Fuse Selection
The switch on the rear of the 2105 is used to configure the power analyzer for operation at
different AC line voltages. The supply voltages and their corresponding fuses are listed below:
105 to 125 VAC, 50-400Hz = .250 Amp Slo-Blo
210 to 250 VAC, 50-400Hz = .125 Amp Slo-Blo
Note that this switch is for the internal operating voltages required by the power analyzer, and
should not be confused with the input voltage selectors on the front of the instrument.
2.4 Bench Use
The 2105 is delivered for operation in bench use and special instructions for use in this manner
other than the procedures of Section 4 are not required.
10
2.5 Rack Mounting
An optional kit is available for mounting the 2105 in a standard 19" equipment rack. This is
listed in Section 4 of this manual. Follow the installation diagram included with the kit. If the
2105 is to be transported while mounted in a rack then it must be supported so as to prevent
upward and downward movement.
The user should note that the specifications for the 2105 become degraded at high
temperatures thus it is required that sufficient room be allowed for airflow around the 2105.
This may be achieved by placing a minimum 1.75" blank panel above and below the 2105 in the
rack.
If a unit placed beneath the 2105 has an unusually hot exterior top surface and it is not possible
to alter its location, it is recommended that an aluminum "reflector" plate be used between this
unit and the 2105.
Under no circumstances should the ambient air temperature surrounding the 2105 be allowed
to exceed 50°C while in operation or 70°C while in storage.
2.6 Safety Precautions
The power connector is a three-contact device and should be mated only with a three-contact
connector where the third contact provides a continuous ground connection. A mating power
cord has been provided. If the power is provided through an extension cable then the ground
connection must be continuous throughout this cable. Failure to provide a continuous ground
connection to the 2105 may render it unsafe for use!
Lethal voltages are routinely present on the rear terminals of the 2105! Always disable the
power source before changing load or source connections.
11
Chapter 3 Specifications
Current Ranges
.2000A
2.000A
20.00A
30.00V
6.000W
60.00W
600.0W
150.00V
30.00W
300.0W
3000W
300.0V
60.00W
600.0W
6000W
600.0V
120.00W
1200.0W
12000W
Watts
This section contains accuracy, operating and environmental specifications for the Model 2105.
3.1 Range and Resolution Table
Table 1
Ranges
Voltage
3.2 Accuracies
Specified accuracies are valid for a period of 1 year from the date of calibration at 25° C ±5°C,
following a 30 minute warm-up.
Voltage - AC+DC, DC Coupled
DC and 40Hz - 5 kHz: ................................................................................ ±0.1% of reading ±6 digits
5 kHz - 10 kHz: ............................................................................... ±0.5% of reading ±0.5% of range
10 kHz - 20 kHz: ................................................................................... ±1% of reading ±1% of range
(Useable above 20 kHz to 50 kHz with typically an additional 1% error per 10 kHz)
Current - AC+DC, DC Coupled
DC and 40Hz - 5 kHz: ............................................................................... ± 0.1% of reading ±6 digits
5 kHz - 10 kHz: ............................................ ±0.5% of reading ±0.5% of range (12 Amps maximum)
10 kHz - 20 kHz: .................................................. ±1% of reading ±1% of range (2 Amps maximum)
(Useable above 20 kHz to 50 kHz with typically an additional 1% error per 10 kHz)
Watts - True Power (EI A cos φ)
DC and 40Hz - 5 kHz: .............................................................................. ±0.25% of reading ±6 digits
5 kHz - 10 kHz: ............................................ ±0.5% of reading ±0.5% of range (12 Amps maximum)
10 kHz - 20 kHz: .................................................. ±1% of reading ±1% of range (2 Amps maximum)
(Useable above 20 kHz to 50 kHz with typically an additional 1% error per 10 kHz)
12
3.3 Operating Specifications
Crest Factor Response: 50:1 for minimum RMS input, linearly decreasing to 2.5:1 for full
scale RMS input
Minimum Inputs: 5% of voltage and current ranges for specified accuracies
Maximum Voltage Input: 600VDC or RMS, ±1500V
PEAK
(Without damage)
Maximum Current Input: ±35A
, 20ADC or RMS continuous; 100ADC or RMS for 16
PEAK
milliseconds without damage
Voltage Input Impedance: 600kΩ
Current Shunt Impedance: 0.01Ω
Max Common Mode: ±1500V peak, neutral to earth
Peak Indicators: Illuminate at 2.5 x full scale for voltage and current
Over-range: 150% of full scale for DC, up to "maximum input" specification
3.4 Environmental and Physical Specifications
Temperature Range: 0°C to 50°C operating; -20°C to 70°C storage
Temperature Coefficient: ±0.025% of range per °C from 0°C-20°C and 30°C-50°C
Power Consumption: 105-125VAC or 210-250VAC, 50-400Hz; 25VA maximum
Dimensions: 25cm W x 27cm D x 8cm H (10" W x 10.5" D x 3" H)
Weights: 1.7kg (3.5 lbs) net; 3kg (6 lbs) shipping
Source/Load Connections: 4-terminal heavy-duty input jacks
13
Chapter 4 Optional Equipment
This section lists the options available for the 2105 Digital Power Analyzer. Standard
accessories include a detachable power cord and an operation manual.
4.1 Optional Accessories
Option I-100: Current Transformer
This "clamp-on" type current transformer extends the AC current measurement capability on the
2105 to 100 amps RMS. The 100:1 output ratio is 2% accurate from 45Hz to 1000Hz. The device
accommodates up to 2" diameter conductors.
Option I-150: Current Transformer
This "clamp-on" type current transformer extends the AC current measurement capability on the
2105 to 150 amps RMS. The 1000:1 output ratio is 2% accurate from 50Hz to 60Hz, and 3%
accurate at 60Hz to 10 kHz. The device accommodates up to 2 " diameter conductors.
Option I-1000: Current Transformer
This "clamp-on" type current transformer extends the AC current measurement capability on the
2105 to 1000 amps RMS. The 1000:1 output ratio is 2% accurate from 50Hz to 1000Hz. The
device accommodates up to 2" diameter conductors.
Option X21: Load Power Adaptor Cord
This cable is specifically designed for use with the 2105 Power Analyzer. It allows for quick and
easy connection and testing of loads that use a standard AC plug (i.e. televisions, toasters,
microwaves, radios, hair dryers, etc.). The entire cable is 6 feet in length and accommodates
supply currents up to 20 amperes.
Option CC4: Carrying Case
This item is a meter and accessory carrying case designed to protect the Power Analyzer when
moved from one location to another. The case is made of black vinyl and includes a shoulder
strap.
Option R4: Rack Mount Adaptor Kit
This item adapts the Power Analyzer for installation in a standard 19" equipment rack.
14
Chapter 5 Operations
5.1 General
This section of the manual contains complete operating instructions for the Model 2105 Digital
Power Analyzers. Included are control functions, connection methods, and operational
precautions.
5.2 Front Panel Controls
The functions of all front panel controls and indicators are described in the following
paragraphs.
5.2.1 Power Switch
Power is applied to the 2105 by placing the ON/OFF switch in the ON position. Application of
power is indicated by lighting of the digital displays.
5.2.2 V/I Key and Amps/Volts Display Indicator
The Amps/Volts Display Indicator specifies witch value is being displayed on the Amps/Volts
display. The user can switch between volts and amps by pressing the V/I key on the front panel
keypad.
5.2.3 Current Range Keys
These keys select the range of current that will be measured. For the greatest accuracy, select
the range that provides the highest resolution without exceeding the value of the range.
If the amount of current is unknown, select the highest current range before applying power to
the load. If the amount of current can be approximated, select the appropriate range.
5.2.4 Voltage Range Keys
These keys select the range of voltage that will be measured. For the greatest accuracy, select
the range that provides the highest resolution without exceeding the value of the range.
If the level of voltage is unknown, select the highest voltage range before applying power to the
load. If the level of voltage can be approximated, select the appropriate range.
5.2.5 Peak Amp Overload Indicator
This indicator may be used to alert the user to the presence of spikes on the input signal. If the
peak or steady-state current to the measured load is greater than 250% of the range selected
with the Current Range Key, the "Amps O/L" indicator illuminates. Select a higher current
range or reduce the current to the load to extinguish the indicator. Current and power displays
may or may not be correct and should not be trusted if this indicator is lit.
15
5.2.6 Peak Volt Overload Indicator
This indicator may be used to alert the user to the presence of spikes on the input signal. If the
peak or steady-state voltage applied to the load is greater than 250% of the range selected with
the Voltage Range Key, the "Volts O/L" indicator will be illuminated. Select a higher voltage
range or reduce the supply voltage to extinguish the LED. Volts and power displays may or may
not be correct and should not be trusted if this indicator is lit.
5.2.7 Amps-Volts Display
When the Volts Display Indicator is ON, the right-hand digital display will show the True RMS
(AC+DC) voltage applied to the rear panel connectors.
When the Amps Display Indicator is ON, the True RMS (AC+DC) current passing through the
internal shunt will be displayed in the right-hand display.
A flashing display in either mode indicates an overload condition. Reduce inputs or increase the
ranges until a stable reading is observed.
5.2.8 Watts – True Power Display
The power dissipated in the load (in watts) is indicated on the left-hand display. This reading is
invalid if either the amps or volts overload indicator is illuminated. These peak indicators occur
independently of whether "Amps" or "Volts" has been selected for the right-hand display. In
either case, select a higher range or reduce the input signals. If the highest range has been
selected and a peak O/L indicator is still on, the measurement may require the use of external
dividers and/or current shunts.
This display includes a polarity (-) sign which indicates the general phase relationship between
the voltage and current. A negative sign indicates that the voltage and current are out of
phase. The lack of a negative sign means that the voltage and current are in phase. Note that a
negative sign may also indicate that the voltage or current leads are reversed. The polarity sign
may also be used to determine source or load power. Load power is that power which is
consumed by a load. Source power (-) is that power which is supplied by a source.
If external dividers or shunts are used, the power display will be a fraction of the actual load
power. Simple ratio calculations will then reveal the power actually delivered to the load. See
Section 5.4.2.
Under certain conditions where complex waveforms are being applied to the load, it may be
necessary to select a current and/or voltage range that is well above the true RMS value
indicated on the display in order to extinguish the "O/L" indicator. This will reduce the
resolution of the wattmeter reading, but is necessary for instrument accuracy. The display will
blink if the load power is greater than that required for a display of 19999 (decimal omitted).
16
5.3 Rear Panel Controls
The locations of the rear panel controls are shown in Figure 5-1. The function of the 115V/230V
switch and power cord receptacle were covered in section 2-3. It is important to connect the
load in accordance with the L (line) and N (neutral) indications on the rear panel for maximum
safety and accuracy. Refer to section 5-4.
Figure 5-1 | Rear Panel
5.4 Making Connections
The 2105 is designed to accommodate a multitude of connection methods allowing the user or
test engineer to design custom harnesses according to the application. The basic guidelines for
making connections are:
1) The supply voltage is connected between the "L" and "N" terminals (line and neutral).
2) The current to the load passes through the internal shunt. The load is connected to the
right-hand
In effect, the Power Analyzer is placed in series with the load to monitor the power drawn by it.
Some connection methods are more complex and require some explaining. These are
described in the following paragraphs.
CAUTION!
Lethal voltages are routinely connected to the terminals of the 2105. Make sure that power
sources are disabled before making or removing connections.
17
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