Megger LT300 Application Guide

110

V Systems

MFT1700 Series

Testing on 110 V Systems

Application note 25

Testing on 110 V Systems

110 V electrical systems are used in applications which call for
extra safety, such as construction sites. 110 V installations are
centre-tapped to earth meaning that the potential to earth is
halved and the shock hazard is just 55 V. Even though 110 V
systems are safer, they still require the same tests prescribed by
the 17th edition of the wiring regulations BS7671.

Insulation Testing

As directed by BS7671, when performing an insulation test on a
110 V circuit, you should select the 500 V range on your
insulation tester.

Loop Testing

Since 110 V systems are actually 55 V centred tapped to earth,
the operating voltage range of your loop tester has to reach as
low as 55 V in order to perform a test. However, due to losses
of potential in a circuit, the actual voltage could be lower than
55 V. Therefore, to ensure that your tester will still operate if
any voltage drop occurs, its minimum operating voltage should
ideally be 50 V or better still 48 V.

If you have a 110 V RCD protected circuit, only a two-wire low
current/no-trip loop tester, such as an LTW325 or any of the
MFT1700 series, can be used. A 3-wire tester requires the
neutral and earth to be at the same potential therefore could not
be used.

The following diagram shows how a loop tester should be
connected to a 55 V centre-tapped system.

RCD Testing

RCD testers should also have an operating range down to 50 V
so that they may be used on 55 V centred tapped systems.
However, if your RCD tester does not have an operating range
down to 50 V but to 100 V, it is still possible to test the RCD.
This can be achieved by connecting your tester in the method
shown below, it will then read 110 V and will be able to operate.

By connecting the RCD tester across the RCD on the phase and
neutral conductors, the leakage current is then flowing through
the test instrument so the RCD will still detect the imbalance of
the current flowing. Although harder to make the connections,
this is still a valid way of testing an RCD.

Instruments suitable for use on 110 V
systems:

Insulation Testing:
MIT200 Series
MIT300 Series
MIT400 Series (500 V models only)
MFT1700 Series
Loop Testing:
LTW325
LTW335
LTW425
MFT1700 Series (2-wire tests only)
RCD Testing:
RCDT320
RCDT330

UK

Archcliffe Road Dover
CT17 9EN England
T [phone]
F [phone]
[e-mail]

OTHER TECHNICAL SALES OFFICES

Valley Forge USA, College Station USA, Sydney
AUSTRALIA, Täby SWEDEN, Ontario CANADA,
Trappes FRANCE, Oberursel GERMANY, Aargau
SWITZERLAND, Kingdom of BAHRAIN, Mumbai
INDIA, Johannesburg SOUTH AFRICA, Chonburi
THAILAND

UNITED STATES

4271 Bronze Way
Dallas TX 75237-1019 USA
T 800 723 2861 (USA only)
T [phone]
F [phone]
[e-mail]

CERTIFICATION ISO

Registered to ISO 9001:2008 Cert. no. Q 0 9250

Registered to ISO 14001-2004 Cert. no. E MS 61597

110V_AN25_en_V02

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