Simultaneous RCD tripping?

[a546345]

Hi folk,

I had an unusual electrical issue last night. Any help appreciated to work out what happened.

TT earthing system, diagram below. The CU was replaced, I believe, in 2007, and I had some changes made a couple of months ago, described below.

At 1:38am this morning both the RCD socket powering the UPS, and the RCBO for Sockets 1 simultaneously tripped (highlighted on the diagram). I know it was simultaneous, as the log for the UPS, and the log for the alarm both had entries at this time.

When I checked in the morning, the RCD socket for the boiler had also tripped (also highlighted). I don't have anything logging on that circuit so I don't know if it was at the same time as the others.

All the devices resetted ok this morning.

The Socket1 and Socket2 RCBOs (previously MCBs), and the RCD sockets in the boiler room were put in recently by an electrician as well as the time delay main incomer (previously 100mA 30ms). This was to help diagnose any trips, and to get the discrimination between the main incomer, and the various downstream RCD/RCBOs.


Any suggestions as to what could cause trips in this way?

Thanks!

Untitled

• a546345
• 12 Nov 2011
• 1

 

[ericmark]

I have had two RCD's feeding the house since 1990 and it is quite common for both to trip or one to trip as one resets the other.

Spikes due to atmospheric electrical storms are most common reason but often no reason can be found. I would say about once every 6 months one or the other or both will trip. Resetting we find no reason why they tripped.

If one is really worried you can get auto resetting types

Untitled

• ericmark
• 6 Dec 2008
• 34

but at £300 plus I will continue to manually reset.

 

[bernardgreen]

If there is a non mains connection ( network, video audio etc ) between the PC and TV on sockets 1 and equipment on the UPS that links the ground connections of these items. ?

If so then I would suspect a fault to earth in the boiler and/or associated equipment with some of the fault current then passing along the non mains connections to equipment on other RCDs

The filter capacitors of PCs, TVs, UPSs and other network equipment provide a circuit for significant currents to flow between earth and neutral. ( significant in terms of an RCD being 10 or more milliamps ). Via these capacitors and interconnected grounds of equipment an earth fault on one circuit can result in significant fault currents through the RCDs protecting one or more other circuits.

 

[flameport]

and to get the discrimination between the main incomer, and the various downstream RCD/RCBOs.

Are those RCBOs double pole?
If not, certain types of fault will still trip the main 100ma RCD.

 

[a546345]

Thanks for the replies.

The digital alarm clock plugged into Sockets 1 has malfunctioned since the trip. I could believe a spike could have been the cause.

How does a spike present itself to the RCBO/RCD causing it to trip?

The RCBOs are single pole, but the RCD sockets are double pole, precisely to protect against N-E faults.

BernardGreen - good call that could be it. There is a serial connection between the UPS and the PC on Outlet Group 2. There is also a VGA connection from this PC to an LCD monitor (integral psu with IEC C13 cable) plugged into Sockets 1. I think both these connections connect the earths as you describe. The TV aerial is plugged into this PC also.

There are also network connections, but I believe these are floating and do not connect the earths.

I shall replug the monitor into Outlet group 2.

Do you still think there was a fault on the boiler or diverter valve? Any suggestions on how to diagnose?

cheers!

 

[SimonH2]

and to get the discrimination between the main incomer, and the various downstream RCD/RCBOs.

Are those RCBOs double pole?
If not, certain types of fault will still trip the main 100ma RCD.

Actually, except for fairly "mild" faults, there is no discrimination between the RCDs. The fault would need to be enough (>30mA nominal) to trip one while not large enough (>100mA nominal) and so fail to trip the other. In practice (eg someone touching a live part), a fault is likely to go well above the trip current for both devices, and they will both trip.

The incomer would need to be slower than 30ms so as to provide time discrimination from the other downstream devices.

And is it needed anyway ?

 

[a546345]

The main incomer is a time delay type (100ms) so there is discrimination. The reason for putting it in is that I didn't want any earth leakage fault anywhere in the house causing the main incomer to trip. For 2 reasons.

1. I had some circuits that I wanted to be reasonably highly available. Such as the UPS/server, and the electric gate.

2. The 30ms main incomer seemed to be more sensitive than the downstream RCD and RCBOs. I had a couple of occasions where the main incomer had tripped, but no downstream protective devices had tripped. This made diagnosis somewhat more difficult!

 

[JohnW2]

Actually, except for fairly "mild" faults, there is no discrimination between the RCDs. The fault would need to be enough (>30mA nominal) to trip one while not large enough (>100mA nominal) and so fail to trip the other. In practice (eg someone touching a live part), a fault is likely to go well above the trip current for both devices, and they will both trip. The incomer would need to be slower than 30ms so as to provide time discrimination from the other downstream devices.

Indeed so. However, as in the traditional (at least, pre-17th ed. situation) TT arrangement, discrimination was not achieved by differences in trip ratings (IΔn), but by the built-in time delay (minimum break time of 50 ms at 5xIΔn, longer at lower imbalance currents) of the up-front Type S RCD. Even in both RCDs had been 30mA ones, that means that discrmination would probably be likely for imbalance currents up to at least 150mA, obviously with much kore discrimnatory power if the upstream one was 100mA.

I've had up-front 100mA Type S RCDs for nearly 25 years - although now no longer necessary, because all final circuits have RCD/RCBO protection (but retained as isolators' ). In that 25 years, I don't think one of them has ever operated (despite a good few trips of downstream RCDs), except during testing or, on one occasion, when a (single-pole) RCBO-protected circuit developed a N-E 'fault' (which, of course, is expected, since operation of the SP RCBO did not clear the fault).

Kind Regards, John.