PowerBreaker RCD fused spur instead of standard fused spur switch?

[Mikey]

Please can anyone advise if i can use a PowerBreaker RCD fused spur instead of a standard fused spur switch. I understand that my RCD fuse box will cut incase of any fault but would like to replace the standard fused spur switch with the Powerbreaker RCD (Greenbrook) which feeds my outside pond pump and outside socket. In which case if either fail it will just cut out at the Powerbreaker RCD instead of the fusebox. Or must I still combine a standard fused spur switch with the RCD powerbreaker.

[spuorgelgoog]

No, it won't. There is no discrimination between RCDs and either or both will cut out in the event of an earth fault.

Owain

[John Rumm]

Unfortunately if there is excess earth leakage, both RCDs will see it and either or both may trip. Even if they have different trip thresholds, if there is enough leakage to trip the upstream one you will still see one or both trip.

This is a problem known as "discrimination" i.e. the practice of ensuring that only the protective device for the circuit (or part of) close to where the fault condition exists trips, and not ones responsible for other parts of the installation.

The only way to ensure you have discrimination with cascaded RCDs is to use a type with a built in time delay[1] in the upstream part of the circuit.

[1] Usually called a Type S (S for "selective")

[ARW]

And is probably going to be very difficult for the OP to install and meet the regs

[The Natural Philosopher]

That entirely depends on what values and types of RCD are installed

[John Rumm]

Indeed, with the 18th edition requirements that all cable runs have RCD protection unless they are buried >= 50mm from the surface, or be actually wired visibly on the surface, or are protected by earthed metallic screening (conduit, armour etc), its complicated to get a non RCD protected feed to a place where you can sensibly stick your RCD spur.

A split load CU, with a non RCD section, feeding a MCB feeding a dedicated radial circuit wired in earthshield or SWA etc to the spur position, and then RCD connection unit at that point.

[ARW]

Far too over engineered for my liking John:-)

If it's a radial from the CU then you would just fit a RCBO in the CU and not bother with the SWA or earthshield [1] and then just use T&E up to the final connection to the pond.

[1] Do they still sell it? Did they ever sell any? Was it ever rated for 6mm with a 32A MCB? I think I once saw some.

[John Rumm]

Well yes, but it does skip the RCD at the point of use. I was assuming here that the OPs desire was to have any trip occur in a place that does not require a schlep back to the CU to reset it.

Not sure I have ever seen any (earthshield that is) in real life...

[tabbypurr]

I'm not aware of anything preventing you using an RCD fused spur on its own. As said there's no knowing which will trip if a leakage event occurs, but if you've already got the RCD spur I guess that beats 100% chance of it being the CU one to trip. If you were thinking of buying one I doubt it's worth it.

NT

[Mikey]

replying to John Rumm, Mikey wrote: Many thanks everyone. It seems that it is not worth doing then as like you say either or both could trip anyway. I thought as the PowerBreaker RCD fused spur would be the foremost directly linked switch to the sockets that it would trip first instead of the fusebox. The powerbreaker is a passive 30mA but not sure what mA the fusebox would trip at.

[Mikey]

replying to tabbypurr, Mikey wrote: Thanks for your reply. I did had already purchased the Greenbrook 10mA Powerbreaker. I guess I could try it and see which possibly trips first. Is it possibly double safety in the fact that i would have two break points in case of failure?

[John Rumm]

A typically RCD in the CU would also be 30mA trip. However since the RCD has no control over the magnitude of the leakage current, and they are both seeing the same leakage current, there is not way of predicting what will trip in most cases.

[John Rumm]

No, just double the odds of getting a failed RCD or nuisance trip :-)

If you have a 10mA trip device downstream of a 30mA trip device, then there is a narrow window of leakage currents (anything from ~10 to ~20mA) that would trip the "smaller" device in preference.

[Scott]

When I upgraded the consumer unit, I asked for RCBOs instead of RCDs (eight of them). I gave the computer a circuit of its own. Same for alarm system. If you gave the outside electrics a separate RCBO, this would achieve the purpose.

The electrician was shocked (excuse the pun!) when I suggested this as he said it would be far too expensive but when he priced it up it was much cheaper than he expected and he agreed it was the 'way to go'.

[Mike Clarke]

Given that 30mA is a nominal level and tolerances will be different for each it's pot luck which would trip first. But if they both trip at exactly the same current the RCD in the CU could be more likely to trip first since it would also 'see' any small background leakage elsewhere in the circuit(s) which would be in addition to what the local RCD sees

[Mikey]

replying to John Rumm, Mikey wrote: Thanks John, Do you think I should give it a go and replace the standard fused spur switch with the 10mA passive powerbreaker or not bother?

[John Rumm]

I suppose that depends on what are you hoping to achieve?

Will it be "safer", no - the 30mA trip device (if that is what you have[1]) will offer adequate shock protection.

If you are dealing with a circuit that trips often and would like to save a walk back to the CU on some occasions, the the additional one

*might* do that *sometimes*. However you would likely be better off fixing the case of the frequent trips. [1] Normally a RCD in a CU is a 30mA trip device, and that will protect people against the more egregious effects of shock. Some CUs (notably those complying with the 16th edition of the wiring regs) and using TT earthing (i.e. those with no earth provision from the electrical supply) may have 100mA trip RCDs covering all circuits. These are for equipment and fire protection, but do not offer adequate protection for people from electrocution.

[tabbypurr]

alerady been explained why this is pointless

no. why was already explained.

NT

[Mikey]

replying to John Rumm, Mikey wrote: Thanks John. Removed the cable with the earth corrosion showing and all good now.

[John Rumm]

So probably compromised insulation in the cable giving it a low insulation resistance.

(I have a three core SWA in my garden that is showing low IR on one core for no obvious reason - I had to abandon that core in the end. Fortunately it was a lighting circuit with PIR control, and the extra core was just used as an "always on" override)