RCD before or after a UPS?

S

Susan_electrical

Member
Location
United Arab Emirates
Occupation
Electrical Engineer
Hello all
In a UPS Planning Guide by ABB, it is advised not to place a residual current breaker before the UPS, but after it. I'm confused.

An RCB protects against earth leakage faults (downstream), am I correct? Then what difference does it make placing it before or after the UPS? Am I missing something here?
 
Susan_electrical said:
Hello all
In a UPS Planning Guide by ABB, it is advised not to place a residual current breaker before the UPS, but after it. I'm confused.

An RCB protects against earth leakage faults (downstream), am I correct? Then what difference does it make placing it before or after the UPS? Am I missing something here?
Click to expand...

It says your in the Emirates, they use a TT or a TN-S system, which is different than what most on here are used to (TN-C-S).
Presuming your working under the 'The Electricity Wiring Regulations of 2020', (those are almost an exact copy of the International Standard IEC 60364 or BS 7671).
You need to provide protection to people against electric shock (due to Direct or Indirect Contact with electricity) for the final portion of the circuit regardless if the UPS is on or off.
5.1.3 Protection of persons against electric shock due to Direct Contact or Indirect Contact
must be provided by one of the methods detailed in Regulation 5.3.
Click to expand...

A 30ma RCD provides electric shock protection for the end users so that needs to be after the UPS.
You also probably need to provide Earth Leakage Protection (ELP) on the feeder that supplies a UPS, what we call GFPE protection, and that is sizing that is more complicated depending on if your on a TT or TN-S.
Hope that helps
 
tortuga said:
It says your in the Emirates, they use a TT or a TN-S system, which is different than what most on here are used to (TN-C-S).
Presuming your working under the 'The Electricity Wiring Regulations of 2020', (those are almost an exact copy of the International Standard IEC 60364 or BS 7671).
You need to provide protection to people against electric shock (due to Direct or Indirect Contact with electricity) for the final portion of the circuit regardless if the UPS is on or off.


A 30ma RCD provides electric shock protection for the end users so that needs to be after the UPS.
You also probably need to provide Earth Leakage Protection (ELP) on the feeder that supplies a UPS, what we call GFPE protection, and that is sizing that is more complicated depending on if your on a TT or TN-S.
Hope that helps
Click to expand...
Thank you @tortuga . I was wondering why ABB worded it like this
We advise against the use of a residual current circuit breaker BEFORE the UPS, especially so as not to compromise the envisaged protection concept.

My apologies if I sound silly, but, won't placing the RCCB before the UPS protect whatever is downstream (ie the UPS and the load circuits)?
 
Correct. As for "why no RCD before the UPS," that's because nearly any UPS system out there is going to have built-in noise filtering on the input, and that noise gets shunted off to ground, possibly causing nuisance trips of the RCD.
 
If the UPS is regenerating power, an RCD on the line side is not going to protect people on the output side unless the UPS is in bypass mode. Whether you need one on the line side would be driven by your electrical code even it makes sense or not. If RCD protection is required for the output circuits, then you need it on the output side of the UPS. This could possibly be done in its output distribution panel such that it protects whether in normal or bypass mode. This would be the better way. As others pointed out, being on the line side doesn't help much since it will trip (and possibly nuisance trips because of filtering) and the UPS will switch to batteries and may die if no one notices the line side fault. The only reason to use a line side RCD would be because the code requires it, dumb or not.
 
suemarkp said:
If the UPS is regenerating power, an RCD on the line side is not going to protect people on the output side unless the UPS is in bypass mode. Whether you need one on the line side would be driven by your electrical code even it makes sense or not. If RCD protection is required for the output circuits, then you need it on the output side of the UPS. This could possibly be done in its output distribution panel such that it protects whether in normal or bypass mode. This would be the better way. As others pointed out, being on the line side doesn't help much since it will trip (and possibly nuisance trips because of filtering) and the UPS will switch to batteries and may die if no one notices the line side fault. The only reason to use a line side RCD would be because the code requires it, dumb or not.
Click to expand...
Thank you @suemarkp
 
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