RCD trip when power cut occurs

[Sadsyndrome]

Hello. we have had a few powercuts over past few months and am in middle of one at the moment where power has gone off a few times while DNO are trying to resolve issue. House is a mid 1980'S PME and on housing estate.
Current setup is single RCD with mcbs. Am currently waiting for spark to return from his hols to replace for full rcbo set up.

However, in the meantime, can anyone explain why the existing RCD trips at the point of the power cut..ie. not at turn on. I know this happened as it just tripped (my settee is near the RCD and I heard it switch off) when the power went a few minutes ago and neighbour has also said power is on and off a few times whilst the DNO men are trying to fix their fault.
I roughly understand how RCDs work but can't grasp why one would trip at the point of a power cut to the mains incomer?

Can anyone explain for me?

thanks

 

[brianmoooore]

Probably a minor neutral to earth fault somewhere in your wiring. Big surge of power to appliances when power is restored, and a very small proportion of the neutral current passes through the leak and flows through the earth wire instead.
The RCD sees the full live current and the slightly reduced neutral current, which combined with normal capacitive leakage from live to the earth wire, seems to be enough of a difference to switch off your RCD.

 

[Sadsyndrome]

Probably a minor neutral to earth fault somewhere in your wiring. Big surge of power to appliances when power is restored, and a very small proportion of the neutral current passes through the leak and flows through the earth wire instead.
The RCD sees the full live current and the slightly reduced neutral current, which combined with normal capacitive leakage from live to the earth wire, seems to be enough of a difference to switch off your RCD.

Yes thanks, that does make sense. I know my "ambient" total earth leakage is quite high hence swapping over to rcbo. However, this issue was specifically occurring at time of power outage not restore?

 

[happysteve]

The nature of power cuts often means there are very fast (ie high frequency) changes in voltage and current occurring. As well as the cables in your installation themselves having inherent capacitance, many electric filters behave more like short circuits the higher the frequency; these transients are what causes the RCD to trip when there's a power cut.

 

[littlespark]

Just out of curiosity, how many circuits are running through this one RCD?

And for everyone else, what’s the maximum number you’ve found?

Can regularly find 5 or 6 in split boards… Up to 10 sometimes…but the most I’ve ever seen was 20 in an MK sentry.
Couldn’t keep all 4 of the lighting circuits going… mostly LED downlights. Any combination of 3, but not all 4.

 

[timhoward]

And for everyone else, what’s the maximum number you’ve found?

RCD Main switch MK with about 10.
Initially the test button didn’t work, then when manually operated it started working….and wouldn’t stay on!
One of those great situations where you’ve been in the house 10 minutes and ‘broken everything!’

 

[Sadsyndrome]

⁸
My current installation has 6 (ring,2xlight,cooker,hob,imm/gar)
3 bed semi 7mx5mbuilt (built 1986) + 6mx3m extension(built1993)
Do you have any idea why rcd trips when power outage occurs?

 

[happysteve]

⁸
My current installation has 6 (ring,2xlight,cooker,hob,imm/gar)
3 bed semi 7mx5mbuilt (built 1986) + 6mx3m extension(built1993)
Do you have any idea why rcd trips when power outage occurs?

Are you asking:

(1) why your RCD trips when there's a power cut?
(2) why your RCD trips when there's a power cut?

The answer to (1) is in post #4 above:

"The nature of power cuts often means there are very fast (ie high frequency) changes in voltage and current occurring. As well as the cables in your installation themselves having inherent capacitance, many electric filters behave more like short circuits the higher the frequency; these transients are what causes the RCD to trip when there's a power cut."

The answer to (2) is a combination of the following:

- the nature of the circuits (types and lengths of cable) affecting the amount of inherent capacitance
- things with filters that might behave differently to high frequency glitches (like a power cut) than they do to 50Hz, e.g. desktop PCs, washing machines, dishwashers, etc)
- amount of existing earth leakage (LED lights, desktop PCs etc)
- the trip characteristics of your RCD (does it trip at 17mA or 24mA?)

You're not alone in this happening. It happened to my in-laws a couple of years ago, when there were local DNO infrastrucure problems and frequent power cuts. They have a DB with 2x RCDs, sensibly split, house only 10 years old, all new appliances, all insulation tests very high (both with and without appliances plugged in) and RCDs working well (with identical responses) on all tests... yet one RCD, the same one, would always trip when there was a power cut, and the other one wouldn't.

Never got to the bottom of why. The DNO fixed their network, it's been fine since.

 

[pc1966]

all insulation tests very high (both with and without appliances plugged in) and RCDs working well (with identical responses) on all tests... yet one RCD, the same one, would always trip when there was a power cut, and the other one wouldn't.

Probably one circuit had one or (most likely) several bit of equipment with more capacitive filtering from L/N to E. I would guess something like a washing machine with VFD for the motor, etc.

As above, RCD trip on current imbalance, and that is I = C * dV/dt probably dominated by the line conductor (as the power is switched) along with the capacitance 'C' of line to earth/CPC of all connected circuits.