Outgoing Feeder RCBO Trips During UPS Normal Source Restoration

thankyou

The RCBO can trip for three reasons:

1. A fault giving rise to overcurrent

2. Earth leakage

3. Faulty Circuit Breaker

4. Inrush current

You ruled out earth leakage by testing those three circuits. Also, possibility of fault or earth leakage happening in three circuits simultaneously is very very unusual and hence can be ruled out.

Faulty Circuit breaker is a possibility, but in three circuits simultaneously???

Inrush current is possible depending on the type of load, the status of UPS voltage (in other words what is the remaining capacity of the UPS battery) just before mains was restored - you may like to look into. I presumed the trip took place just when the mains was switched on. Also, I presume the battery is 10 years old and hence could have capacity issues.

thankyou

What phase(s) were the three offending breakers connected to ?

Why are RCBOs applied in this case?

thank you, 2 loads were in r phase, 1 was in blue phase, rcbos are always preferred for critical loads because if we provide common elcb for the complete db, the complete db will be failed during earth leakage in any one of the circuits. so ups are always essential load hence rcbos.

It would be interesting to find out if those 3 circuits have any particular distinction over the others, cable construction, length, loads served. You could perhaps move the breakers to see if the problem follows the breaker, or the circuit.

I expect that the tripping is likely to happen again, and maybe has something to do with your particular RCBO sensor circuit and the THD (Total Harmonic Distortion) of your UPS inverter, possibly 3rd harmonic. The charging capacitance of your cables, the phase firing at just the right time, is possibly enough to produce apparent current in the return leg that is not sensed in the supply. This is a common occurrence when long cables are used with Ground Fault Circuit Interrupters in the US, or newer Inverter Generators, engine driven inverter used to reduce part load fuel consumption. Cable capacitance is the driving factor.

Assuming your power system is solidly grounded, not in a wet area frequented by people, any circuit breaker will easily clear a ground fault. You don’t need an RCBO to clear a ground fault from your panel. Your breaker is there to protect the wire, and unless you have a demonstrated risk of an arcing ground fault in your cables returning outside of the cable jacket, RCBOs may be technical over kill. Maybe you can do a formal risk analysis, and decide that the benefit of the over-application of technology comes with little to no benefit.

If you are uncomfortable with perhaps overriding your local codes, then I would investigate replacement RCBOs that use a different technology to sense ground fault current.

I'm kind of wondering why the battery bank's condition has not been first established, validated and ascertained as part of the weekly routine testing or PM process prior to disconnecting the UPS from the mains...

I believe that should be the very first step as part of your QA processes..

battery bank voltage usually checked in routine test and noted. there was nothing abnormal.

Battery charge condition changes and can be noted when and while in a fully loaded condition.

As the input power source, any imbalance especially in a polyphase application can produce unpredictable output behavior when fully loaded.

Please enlighten as to how an <...imbalance...> can cause such current in the circuit protective conductor as to cause <...Outgoing Feeder RCBO Trips....>?

Basically 3 single phase inverters are used, wired and configured for 3-phase operation to provide uninterrupted source of power.. and each inverter is backed by a DC source to ensure continues operations.. The battery banks in turn are being maintained and kept charged by the commercially available power source..

Hence as a result any imbalance in the battery bank conditions will be reflected onto the converted power output of the 3 phase system..that can trigger any of the circuit breakers to trip..

Isn’t it likely that there is only one battery bank?

Using a single battery bank power supply I believe would be appropriate depending upon the load applications and UPS rated power handling capacity..

The question of current in the circuit protective conductor remains unresolved, then.

In my opinion any minor UPS phase voltage differences at the input side in my view can be reflected as current imbalance at the output side which may also be just enough to trigger the tripping of the circuit breaker...

It is difficult not to do any speculation since the actual circuit diagram and load distribution for the phases were not provided by the OP..

The 3 phase currents may be unbalanced, but a single phase circuit, either line to line or line to neutral would not be able to detect it. Apparent current due to charging of cable distributed capacitance to ground can fool the sensors. But occurring once in a great while is the mystery.

By definition, <...a single phase circuit...> fed from a 3-phase supply is <...unbalanced...>. The query is that of why imbalance would cause an <...RCBO...> to trip on earth/ground current, which needs to be expressed meaningfully. It is commonplace for <...RCBO...>s to be installed on single phase circuits without tripping on earth/ground current.

yes, i think so, if its a 3phase rcbo there is a chance of tripping due to unbalance. but here all the outgoings are single phase only

Single phase loading distributions and scheduling may need to be factored in and considered so as not to affect and introduce much imbalance to the 3phase inverter system.

Unbalanced phase loads will not trip a 3 phase RCBO, whether 3 wire or 4 wire design, unless there are significant harmonics not accounted for in the breaker design.

All load currents are sensed and summed.

In a TN-C-S system, for example, it is normal to have current in the circuit protective conductor if a neutral conductor upstream of the installation is broken. Such currents present <...RCBO...>s with the conditions needed to trip on earth current.

That is why the services of a qualified local Electrician would be of value here. There is a lot that needs investigating.

Diverted Neutral Current Demonstration - Normal, Open CNE, Current Circulation (Part 2) (youtube.com)

Other than in US dryers and stoves grandfathered in, where is TN-C-S a good idea? That YouTube video without a wiring diagram was not very helpful. Did I miss it?

The <...wiring diagram...> is in part 1.

Question the need for <...RCBO...>s on the downstream circuits if all that is being fed are <...IT servers...>. As well as overload the <...RCBO...> is there for personnel protection and if there is no risk of that prospect being needed then identically rated miniature circuit breakers [MCB] may be appropriate substitutes.

It may be that, if similar behaviour is exhibited by a temporarily-installed MCB, then there needs to be a review of the loads, the cabling, and the MCB characteristics for the purposes of selecting appropriately-rated circuit protective devices for the circuits in question.

If in doubt, consult a local qualified Electrician.