RCD "blinding" by DC

pc1966 · Mar 10, 2021

nicebutdim said:
1. If my understanding is correct, it would hypothetically fine to use Type A time delayed protection upfront of Type AC devices or Type B time delayed (in a future where they exist) upfront of Type A or AC devices.

You can, and the up-front RCD will meet every specification for it.

But usually the bigger concern is the additional shock protection of the down-stream 30mA RCD/RCBO, you would be better to have it the other way round, with the up-front RCD being type AC/A and the downstream being the type A/B so they provide better in-specification tripping on faults with a DC component, is it might be a person that is on the receiving end of the stray current!.

nicebutdim said:
2. The Type AC time delayed RCD I place upstream of my parent's shiny new board, populated with Type A 1P+N RCBOs is far from ideal. This occurred to me after asking questions in the other thread, so I might as well admit this oversight. Not an immediate issue as those RCBOs are functioning as the manufacturer intended (at least they were prior to this lockdown), and there are very few devices in their house capable of causing DC current issues, but it needs to be corrected

No, I would say for a typical TT install with a good earth rod it is not such a concern.

Yes, a type A up-front is better in the sense you have the manufacturer's guarantee that it meets the trip current/times with some DC component present, but my experimental results backs up my expectation that having some small DC component (few tens of mA or so) is not serious, and beyond that an AC/A RCD will always trip if sufficient AC component is present so the current transformer gets enough of a fault signal.

So to back up the RCBO in the case of electronics failing and not wanting the earth system to go to 230V as the rod Ra won't disconnect based on over-current, then the AC up-front RCD will do its job short of a massive DC fault.

Such as DC fault is of course possible in many electronic systems with stored charge, but really only a big risk in my mind from PV generation or EV charging where you have lots of fairly high DC volts outside. For those cases a downstream (final circuit) type B RCD would allow safe disconnection and stop the large DC component, so the up-front RCD would still be able to deal with any AC faults above the specified 100mA/300mA/etc that might be needed for high safety (no single point of failure) TT installations.

nicebutdim · Mar 10, 2021

I'll probably change that upfront RCD just to keep things right, although the chances of any significant DC fault in their home is currently nil.

HappyHippyDad · Mar 10, 2021

pc1966 said:
A recent thread was discussing having a delay RCD (type A or AC) for TT before a garage CU with a type B RCD for an EV charger. The question was would this be OK or not? Standard advice is the type B should be direct off supply (possible problem for TT) or fed from another delay type B (at eye-watering expense). But just how sensitive are type A RCD to a small DC component, below the type B trip point, in practice?

So from the department of "don't try this at home girls & boys" I bring you some test results based on this set-up:
View attachment 65425
Test devices are unused, a year old Hager ADA106U RCBO, and a many years old PowerBreaker H92MPS RCD-FCU. The big potentiometer served to provide a resistance for the current-limited PSU to reliably run at constant current when the RCD was closed, and a path for any mains test current when the RCD opened so it would not be totally forced back through the DC PSU. For testing at home (TN-C-S with RCBOs) I used a simulated TN-C-S test rig so the PSU negative and the MFT earth both go to the pre-RCD mains neutral.

Basically I force DC through the neutral path of the RCD while doing normal MFT tests.

TL;DR Testing RCD to DC effects

Excellent work PC, how long did it take from starting to set up the equipment to finishing the tests?

You just need to do it 100 more times now to increase your statistical accuracy ?

It's made me change my upcoming install from a type AC time delay RCD to a type A, as I have a type A RCBO for the EV charger. Even though the data kind of makes it seem unlikely to blind the upstream RCD.

pc1966 · Mar 10, 2021

happyhippydad said:
Excellent work PC, how long did it take from starting to set up the equipment to finishing the tests?

About 1-2 hours in total, including stuffing results in a spreadsheet to copy & paste here in a neat manner.

happyhippydad said:
You just need to do it 100 more times now to increase your statistical accuracy ?

I was quite surprised by how stable the results generally were so stopped at 3 sets of measurements each, only the near-failing RCD-FCU was a bit unstable on trip-time at 'In' value!

happyhippydad said:
It's made me change my upcoming install from a type AC time delay RCD to a type A, as I have a type A RCBO for the EV charger. Even though the data kind of makes it seem unlikely to blind the upstream RCD.

Certainly I would always go with type A now as the default choice. While AC and A are probably very similar inside, at least the type A has been characterised by the manufacturer for such use.

If inspecting and seeing an AC incomer with type A RCDs/RCBO though I would not be too bothered, I would be a little more concerned by AC for general purpose "additional protection" though, and more worried about EV/PV not using type B.