UPS Overload... RCD Breaking

[FattyMatty]

Hello,

Quick Question, wel quite long actually

I have 2 quite large UPS's at my office, each connected to one 240v 13A double socket on a ring main protected by a 32A MCB. These are the only two devices on the ring main.

There is then an RCD that protectes all of the MCB's in the consumer unit (80A RCD 30mA Trip) apart from the two lighting circuits and the alarm which are on the other side of the split load. The whole lot is isolated by a 100A 2pole Isolator.



When I test the UPS's by switching them off at the wall, they kick in and run properly and the whole building is fine. When I switch then back on, the RCD in the consumer unit trips cutting power to everything but the lights. I then have to run as fast as I can down 4 flights of stairs into the celler to reset the RCD before the UPS's run out of battery power. When I reset it everything is fine and the UPS's start charging again. However the 32A MCB protecting the ring main does NOT trip atall. Hmmmmm....


So I am guessing that when the UPS's come back online, they take a surge of power which trips the RCD?

But why doesnt the 32A MCB inbetween trip first?

Why does the 80A RCD trip?


I was thinking of getting the site electrician to move the 32A MCB over to the non RCD protected side of the consumer unit? Is this wise? or should we use a RCBO?


It would be excellent if anyone could help

Thankyou Very Much

Matthew

 

[ricicle]

Just to clear something up first, the 32A MCB will not trip on a small earth leakage. The 30mA RCD is designed to trip when it detects a leakage to earth of around 30mA.The 80A rating on it is just its full load current handling capability.
I suspect that your large UPSs have a mains filter on them that leak a small current to earth (as is the norm).This could be a little larger on power up and is triggering the RCD.
You could get a sparky to check the installation but if it is the UPSs then you might have to think about having them on the non-RCD side of the board.Then make it known that any sockets they supply must not be used to supply equipment outdoors (label)

 

[davelx]

If there are no other sockets on that ring main, maybe wire the UPS permanently into the ring main with FCUs, then move the ring to the non RCD side of the CU and put RCDs downstream of the UPS's to protect the sockets the equipment is plugged into.

Ideally wire two separate 16A radials for the two UPS's with downstream RCD protection on the equipment circuits

 

[Steve]

To clear up,

RCDs detect EARTH LEAKAGE of a few milliamps

MCBs and fuses detect OVERLOADS of many amps over their rating

This is why an RCD can trip yet the MCB doesnt.

An RCBO does both of the above, which can make fault-finding and diagnosis difficult!

 

[dw3101]

How many KVA are the UPS's?

What are connected to them?

Do the UPS report any overload's, brownouts etc in the logs?

David

 

[tim west]

RCD's react to imbalance if the UPS's are fairly hefty ones as you suggest then there may be an imbalance(lag) between Live and Neutral just long enough to trip it without any earth leakage.

 

[FattyMatty]

Cheers Everyone!

So from all of that ive concluded that the best thng to do is to swap the 32A MCB for a 20A and move it over to the non RCD protected side of the consumer unit.

Im changing to 20A as these are the only two sockets on the ring that will ever be used, and my equipment should never draw more that 16A so a 20A RCD should give me some added protection over the now overrated 32A.

 

[RF Lighting]

If the UPS contains a large transformer then you might need to fit a type 'C' 20A MCB to overcome the inrush current, but you need to check the Zs of the circuit first.