RCD tripping problem

[Mike2007]

I have split board with two rcd's on it and main isolator.
One rcd kept tripping was not letting power through, it latched up but there was no power on top terminals?
After turning off all circuits the rcd would still remain up but no power would pass through, with all mcb's off?
Turned everything off in house and found that I have a knackered table light (sockets) as this caused rcd trip.
My question. How could light cause the rcd fault if all my circuits were off (only earth neutral connected )

 

[bernardgreen]

Neutral to earth faults will trip RCDs even if the live is isolated by the MCB for that circuit.

In some circumstances an RCD will trip on a neutral to earth fault even if there is no live supply to the property.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

is when there is a live supplying a load in the property

If there is a neutral to GROUND fault in a property and the network neutral is a few volts above GROUND then current can flow from network neutral through the RCD sense coil to the final circuit neutral, then through the fault to GROUND. In the perfect installation without any paralled paths to GROUND this is un-likely. It is more likely to happen where there are low impedance parallel paths to GROUND that by-pass the MET or in a TT install.

 

[Mike2007]

Neutral to earth faults will trip RCDs even if the live is isolated by the MCB for that circuit.

In some circumstances an RCD will trip on a neutral to earth fault even if there is no live supply to the property.

Neutral_Earth_trip

• bernardgreen
• 16 Jan 2011
• 48

Thanks for that reply.
What voltage could be expected neutral to earth on single phase domestic?

 

[bernardgreen]

What voltage could be expected neutral to earth on single phase domestic?

The expected voltage should be about zero in normal circumstances, 2 or 3 volts would be tolerable ( Westie may know the acceptable limits.

It is when there is severe un-balance on the loads on the three phases in surrounding properties there will be current along the network neutral and the voltage drop along the neutral will raise the neutral above ground. It can also happen when there is a fault on the network. I have measured over 20 volts during a network fault.

One cause of transient voltage spikes on the neutral is a large ( large for domestic ) motor starting where the starting current is many times the rated running current.

 

[Porque223]

While we're on this subject does anyone have a link for the current vs. time curve for an RCD? Higher currents should take less time to trip.

 

[ericmark]

While we're on this subject does anyone have a link for the current vs. time curve for an RCD? Higher currents should take less time to trip.

Maximum time at 5 times rated current is 40ms and it must trip at rated current 415.1.1 only says it must trip at 30ma it does not say how quickly only at 5 times rated current is there a time stated.

In practice I have not found many 30ma RCD's which will not trip within the 40ms at 30ma the last few I tested were tripping at 10ms but active RCD's do tend to be a little quicker than the passive type.

There are some RCD's which instead of tripping between 50% and 100% of rated current trip at between 90% and 100% (X-Pole). The same RCD also has a pre warning so one is alerted as the leakage approach the limit. There are even auto resetting RCD's been some time since I read the spec on them.

Getting 110 volt RCD's seems to be a problem in the UK. There are some in-line types for sale but the problem is either they have to be active type or able to work when there is a fault taking up to the full power permitted by the over current device. To ensure there is enough voltage for them to work the loop impedance would need to be extremely low and with most of our transformers being 3Kva or less in real terms only active types could be used. Clearly in the USA the transformers are much larger but to try to compare USA and UK systems is not really going to help.

I would look on a USA forum.

 

[RF Lighting]

I thought it was a maximum of 200mS at the rated current?

 

[JohnW2]

Maximum time at 5 times rated current is 40ms and it must trip at rated current 415.1.1 only says it must trip at 30ma it does not say how quickly only at 5 times rated current is there a time stated.

Whilst that's true of 415.1.1, Table 3A in Appendix 3 gives you the information you are 'missing' ... an RCD is required to trip within 300ms at its rated current, within 150ms at 2 x rated current and, as you say, within 40ms at 5 x rated current.

Kind Regards, John

 

[ericmark]

I thought it was a maximum of 200mS at the rated current?

I also thought there was a limit quoted but think it must be one of the rule of thumb things as can't find any time quoted for 30ma only 150ma which is 40ms.

Also of course the 40ms rule is only for the 30ma RCD with the 300ma RCD I can find no time limit. Looking at table 41.2 I would have thought the RCD should trip within the 400ms the same as the fuse although 411.4.8 refers to 5 seconds.

For non final circuits the times can be far higher on building sites I have worked on the MDU was set to 5 mins the next to 1 min and so on.

I am uncertain of the real answer here but would want to read it in the BS7671 not any of the guides.

 

[JohnW2]

Also of course the 40ms rule is only for the 30ma RCD with the 300ma RCD I can find no time limit.

As I said, look at Table 3A in Appendix 3. The maximum trip times I quoted (300ms at rated, 150ms at 2 x rated and 40ms at 5 x rated) apply to RCDs rated at 10, 30, 100, 300 or 500 mA.

For 100mA, 300mA or 500mA Type S (time-delayed) RCDs, the acceptable trip times are 130-500ms at rated cirrent, 60-200ms at 2 x rated and 40-150ms at 5 x rated.

Kind Regards, John

 

[ericmark]

Thanks as you rightly say Appendix 3 Table 3A shows 300ms max for 30ma RCD this it would seem is an exert from BS EN 61008-1 and BS EN 61009-1.

It would seem however if I am reading that table correctly if it takes over 150ms at 30ma then you would have to also test at 2X rated current.

So this is the answer to question.
X1 = 300ms
X2 = 150ms
X5 = 40ms
Only three points to curve but there is a sort of curve.

 

[JohnW2]

It would seem however if I am reading that table correctly if it takes over 150ms at 30ma then you would have to also test at 2X rated current.

Well, yes, of course - and similarly, if it didn't trip within 40ms at rated current or (2 x rated current), then you would also have to test at (5 x rated current). I presume it has to satisfy all three requirements.

Kind Regards, John

 

[EFLImpudence]

It's not really a current/time curve.

Those are the maximum times allowed.

All times are normally under 40ms at all ratings.

 

[EFLImpudence]

It would seem however if I am reading that table correctly if it takes over 150ms at 30ma then you would have to also test at 2X rated current.

No.

Under 300ms @ 30mA is acceptable.

The testers don't do 2x, unless the figures coincide.

 

[JohnW2]

It's not really a current/time curve. Those are the maximum times allowed. All times are normally under 40ms at all ratings.

Indeed - and, in my limited experience, not only are they usually all under 40 ms, but they are also often pretty similar (at 1x, 2x and 3x rated trip current). If that is the general experience, the partial answer to Porque223's question is therefore that the curve appears to be very flat over that range of current inbalances.

I guess that makes sense. 30mA or so may well be about the fastest that the device can physically operate. If it achieves that at rated current, it then can't get significantly faster at higher currents.

Kind Regards, John