RCD "blinding" by DC

[pc1966]

As @newfutile has just said, you would need a very peculiar fault to allow a typical SELV supply to be driving N-E and not go bang! very shortly after.

Anyway for the purpose of the original discussion, that would a 30mA B-type RCD allow enough current to upset a 100mA incomer, the answer is very definitely no. Certainly so if the earth rod Ra will allow for much more than 100mA to flow as you are interested in hard-fault disconnection, not shock protection as such.

However, what is also clear is for 30mA shock protection you can't allow more than a 100mA or so of DC to be present so if the EV charger or PV inverter says 'type B' is needed, then it matters.

 

[pc1966]

The power breaker rcd will be type ac I expect?

It is pretty old (over 10 years at least) and does not explicitly say what type it is.

The data sheet I found on line for the nearest equivalent says "pulsed DC" which would be type A, but when I tested its in-use brothers a few months back using a 'type A' waveform it failed on In/2 by tripping (one way of 0/180 deg, but not the other, forget which).

So yes, I suspect it would be considered type AC.

 

[pc1966]

I also did high current ramp-test of the Hager RCBO for comparison, one data point looks out of place, but my MFT was complaining of being too hot by then to re-test!

Results below show less initial problems with the DC (as seen in the tables earlier) for the RCBO but once you get to higher DC bias it is essentially the same sort of slope of ramp-test trip current versus DC bias current as the RCD-FCU showed.

It might also be my MFT only has 50mA step sizes for ramp testing by then and it just falls on the threshold for the outlying point:

 

[Risteard]

@pc1966 do you have any type AC devices to compare against?

So from how I read it even type A devices here are being paralyzed by DC beyond a few mA?

A Type A is only designed to deal with pulsating DC currents up to 6mA, so anything beyond that and you're on your own! That's why the 6mA DC leakage detection and disconnection threshold is there for the use of a Type A RCD.

 

[nicebutdim]

The thread that begat this thread spawned all sorts of questions in my amateur mind. Most were fairly straightforward and easy enough to reconcile in my amateur mind, but the issue of discrimination between 'types' of earth leakage protection devices was one which I could not reconcile.

Two things I've taken away from all of this are:

1. If my understanding is correct, it would hypothetcially 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.

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

Thanks for taking the time to investigate this further, @pc1966 ?



Edit: Note to any mods reading, the esteemed badge really needs to be removed from my profile.

 

[pc1966]

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!.

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]

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]

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]

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.

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!

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.

 

[Lister1987]

JW does a vid on it;

SparkyNinja does a vid;

There are others bit these have rigs and demos ?

 

[pc1966]

SparkyNinja does a vid;

That was interesting to see it desensitised by 50mA, much worse than either of the two RCDs I had to hand!