SITOP power supply
Electronic protection and fast fault localization in 24 V DC load circuits
SITOP SEL1200, SEL1400, PSE200U selectivity modules
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Edition |
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Brochure |
01/2020 |
usa.siemens.com/sitop |
The SITOP selectivity modules distribute the load current across several 24 V DC load circuits and monitor them reliably for overload and short-circuit conditions. The electronics permit brief current peaks caused, for example, by high inrush currents, but isolate load circuits in the event of an extended overload. This is ensured even on highresistance lines. In such cases, miniature circuit breakers fail to trip, or trip too late, even if the power
supply unit could deliver the required tripping current. The SITOP expansion modules continue to reliably supply 24 V to the load circuits not affected by an overload and can prevent a total system failure.
The new SITOP SEL selectivity modules, which have more outputs, an additional tripping characteristic and detailed diagnostics, now offer even more efficient protection for your plant.
Miniature circuit breakers with high current consumption
Miniature circuit breakers are often still used for the selective protection of 24 V DC load circuits. In many cases, however, when interacting with switched-mode power supply units, they do not offer reliable protection. They require several times the rated current in order to trip within a few milliseconds. Because stabilized power supplies limit their output current electronically when there is a critical overload, the tripping current is not always guaranteed. This can cause the 24-V supply to break in and the PLC to go to Stop. Even if the power supply unit could supply the current, immediate tripping is not necessarily assured, because with this high current requirement, the line resistance can no longer be neglected. It prevents the required tripping current from flowing. Therefore fast tripping is only possible up to certain cable lengths and starting from larger cable cross-sections. In addition to the line resistance, the overall circuit design (e.g. contact resistances at terminals) must be taken into consideration when configuring miniature circuit breakers.
SITOP selectivity modules – optimized for switched-mode power supply units
The selectivity modules are specially designed to protect 24 V DC individual load circuits supplied by switched-mode
power supplies. Individual setting of the tripping current allows optimum adaptation to the respective load circuit. Engineering effort is minimal since the switch-off characteristic always guarantees reliable tripping – even with high line impedances that limit the short-circuit current.
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24 V DC
PLC |
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HMI |
Actor/Sensor |
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As an electronic monitor, the SITOP selectivity module switches faulty 24 V DC load circuits off immediately, and continues to supply the other loads without any interruption.
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SITOP PSE200U and SEL1400 – limiting characteristic offers maximum safety
In terms of the tripping characteristic, the new 8-channel SEL1400 modules behave like the tried and tested 4-channel PSE200U modules. They limit the output current to 150% of the set value, corresponding to the overload behavior of the SITOP power supplies of the Advanced and Standard product lines. This means that, even in the case of a short circuit, no overload can occur and consequently no voltage drop at the output of the power supply unit.
Even when a power supply without overload reserves is used, the patented SITOP concept ensures reliable protection: The electronics continuously monitor the 24 V DC input voltage. As soon as the 24 V DC threatens to fail, the path with a higher current than the set current is disconnected immediately. All other load circuits continue to be supplied without interruption. Even loads that do not comply with the PLC standard and can only bridge a few milliseconds of undervoltage, continue to operate without any problem.
Tripping characteristics
SITOP SEL1200 – switching characteristic for standard protection
The new SEL1200 8-channel selectivity modules also allow higher, brief overload currents. The higher the current, the faster the output is shut down. It is possible that the power supply is loaded for a few milliseconds with a current significantly higher than that which is set. If the power supply unit only has limited overload reserves, it may result in a momentary drop in the 24 V supply. For loads that comply with the PLC standard, however, this is not critical. For standard applications, the SEL1200 selectivity module therefore offers very efficient protection.
For 24 V loads with a high inrush current, the switching characteristic even offers advantages, since whatever applies at shutdown also applies when switching on. High currents are not limited, but can be permitted for a short time. In addition, two outputs of the SITOP SEL1200 or 1400 selectivity modules can be connected in parallel for a 15 A rated current.
Time in seconds
100
10
5
1
0.1
0.01
0
Normal operation
Switch-off after approx. 5 s
Current limitation to 150% for approx. 100 ms, then switching off the faulty output
100% |
150% |
Switching
SITOP SEL1200
Limiting
SITOP SEL1400/PSE200U
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The higher the current, |
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the faster the trip |
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180% 200% |
250% I-set |
SITOP SEL1400 / PSE200U characteristic: limiting
Shutdown response for current requirement per output circuit relative to set value (I/I set value = 100%):
Power requirement: |
Shutdown after: |
From 0 A to 100% (set value) |
No shutdown |
From set value to 150% 1) |
Approx. 5 seconds |
Above 150% 1) of set value |
Current limiting to approx. |
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150% 1) for typ. 100 ms, then |
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shutdown |
Above set value with |
Immediate shutdown |
simultaneous collapse of supply |
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voltage below 20 V DC |
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Startup characteristics: Load ramp-up to 150% of set value with stable voltage supply of at least 20 V
1) SITOP PSE200U versions with NEC Class 2: 110%
SITOP SEL1200 characteristic: switching
Shutdown response: The higher the current, the faster the faulty output is shut down, in line with the limit load integral i2t.
Startup characteristics: Supports loads with high inrush current, whereby the supply voltage can drop to 15 V
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