RCD tripping

[telectrix]

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A greezer in the Farage is worth...?

1 Nigel.

 

[telectrix]

But did the cable supplying the freezer socket need RCD protection?

mine don't. all cabling in garage is surface. not as neat as maybe, but it works.

 

[happysteve]

Not necessarily. SPUs will chop the tops off excessive voltage peaks, but as happysteve explains, what tends to trip RCDs during spikes and brownouts is not the voltage itself but the sudden changes in voltage (dV/dt). The high rate of change causes high peak currents to flow through any capacitance to earth such as within interference suppressors, which the RCD detects as leakage (which technically it is.) Clearly if there is a high peak voltage with a high dV/dt, then the SPD might reduce it to the point where it doesn't cause a trip.

I'd been pondering this for a while and had come to the same conclusion... I almost added some vague thoughts along those lines but lacked the confidence.

So what would prevent this - RCD tripping in due to transients - happening?

(1) less capacitance by design (shorter cable runs/few circuits)
(2) some sort of device that had capacitance to earth before the RCD, so the excess current would flow to earth there rather than in the circuits after the RCD in the event of a transient
(3) a choke (i.e. inductor) to limit the flow of current as the frequency increases

(1) is I guess something to consider when designing from scratch.

I suspect there's some sort of filter that effectively does some sort of combination of (2) and/or (3)... it's just not an SPD. It's the sort of thing you'd buy from RS or Farnell.

 

[nicebutdim]

I assume this would happen to an RCBO?

Can anyone address this question?

Some ideas are running round my head (that I'm not foolish enough to share) and it would be interesting to learn the thoughts of more experienced minds.

 

[Lucien Nunes]

I suppose it's worth considering that any cables / appliances with built-in SPDs to earth, where no main SPD is fitted, will likely cause a trip on a spike. So perhaps there is more advantage to that than I first thought. Of course, it won't help much if any of the integrated varistors have a lower clamping voltage or faster response than the main ones.

Shunt capacitance at the origin, connected via the minimum possible inductance (e.g. feedthrough construction) will soften the edges but at the expense of needing to be pretty durable and hefty caps. Inductors that would provide tangible benefits would be more expensive still. I can't see it being taken up for general installation work, as it's a functionality upgrade rather than a safety one.

 

[John-SJW]

But did the cable supplying the freezer socket need RCD protection?

The cable dropped down in trunking from the CU, under the floor and up to the freezer to a low level concealed socket behind the fridge/freezer. It was not accessible and unlikely to have any future screws run into it, unless they were a foot long.

 

[John-SJW]

Clearly if there is a high peak voltage with a high dV/dt, then the SPD might reduce it to the point where it doesn't cause a trip.

As SPDs appear to be looking to be mandatory soon in new installations. Then they look like they will reduce the problem of RCDs tripping on power on-offs, but fully eliminate it.