DC LEAKAGE EFFECTS

[Eire79]

So after inspection and testing of a domestic property iv identified some remedial works one being the replacement of RCD s to A type from AC due to possibility of dc leakage blocking device from operating. My question is that on a shower priority unit the RCBO is also a AC type and I'm sure this can be left in place as correct me if I'm wrong that there is no possibility of dc leakage on shower circuits , question being has the leakage to be downstream of device or can leakage elsewhere on the installation affect all RCD/RCBO s.

 

[Lucien Nunes]

You are correct - it is DC leakage downstream of the device, i.e. passing through it, that interferes with its operation.

 

[static zap]

DC a hidden dangerous consequence of getting rid of the humble 50 Hz transformer -as something CHEAPER exists .

( Its called change / progress .. And I'm just an old stick in the mud. )

 

[timhoward]

Out of interest was there anything particular in this property that led to this decision such as measuring DC leakage, or particular equipment being present or is it mainly a preventative measure now dangers of type AC devices has come to light?
I'm reflecting that I'm not sure I'd know when this a must and not just a good idea.
I'm also thinking that BPG4 is 6 years old now and probably overdue a revision to mention type AC RCDs.

 

[Julie.]

It's a bit of a complex thing really, if you supply say a car charging itself, if the electronics fail they can inject a DC back into the system. So say it injects just 10A back into the network.

This current would flow into anything and everything connected - so say you have a Ze of 0.35 ohm (assuming the same resistance for phase-N) and a 45A shower connected (shower would be about 5 ohm) - the DC would flow into these two in parallel - simple maths shows it would feed 10A into ~0.33 ohm (.35 & 5 in parallel) they would share this between them, the grid taking around 9.4A and the shower 0.6A - An AC RCD at the supply intake would be blocked, the RCD on the charger would be blocked (if it's not suitable) and quite possibly the RCD on the shower ( if 0.6A is enough to block it).

In this case the effect would be much wider than the circuit itself!

If the DC fed back is higher then more loads would have their RCD blocked - unloaded and light loaded RCDs wouldn't, only the adjacent circuits which are heavily loaded would!

That is why car charging points have to be protected against DC back feed, and lots of issues Re. suitable RCD for them.

This is all ignored by the silly "granny chargers" - they have no such protection, would be plugged into a std RFC with say type AC RCD/RCBO - so any child coming into contact on that same circuit - or potentially other circuits would not have any protection by the RCD.


For the majority of modern loads however, such as laptops and other AC-DC converters there is no appreciable back feed - just sufficient modification of the ac waveform to block the circuit they are on.

 

[loz2754]

I'm also thinking that BPG4 is 6 years old now and probably overdue a revision to mention type AC RCDs.

The most recent revision is July 2019.

 

[timhoward]

The most recent revision is July 2019.

Thanks - only just realised that I've overwritten issue 5 with issue 4 which NAPIT sent me in their welcome pack...!

 

[Lucien Nunes]

say you have a Ze of 0.35 ohm (assuming the same resistance for phase-N) and a 45A shower connected (shower would be about 5 ohm) - the DC would flow into these two in parallel

I am not sure I follow you here. The Ze is in series with a fault passing DC from L-E, while the shower's 5Ω is L-N. Surely what we are concerned with is the DC component of the residual current, for which these two resistances are not in parallel from the point of view of an RCD on a non-faulted circuit serving the shower. If that circuit becomes faulted at the same time as the car is producing DC residual, then yes I agree the shower RCD could be blinded, especially by a hard N-E fault. But the fraction of the DC passing through the shower RCD would then be determined by other unknown resistances e.g. MET to shower circuit fault, rather than shower element and Ze.

 

[Julie.]

I am not sure I follow you here. The Ze is in series with a fault passing DC from L-E, while the shower's 5Ω is L-N. Surely what we are concerned with is the DC component of the residual current, for which these two resistances are not in parallel from the point of view of an RCD on a non-faulted circuit serving the shower. If that circuit becomes faulted at the same time as the car is producing DC residual, then yes I agree the shower RCD could be blinded, especially by a hard N-E fault. But the fraction of the DC passing through the shower RCD would then be determined by other unknown resistances e.g. MET to shower circuit fault, rather than shower element and Ze.

"(assuming the same resistance for phase-N)" - I assume in the above calculation that the measurement of L-E is similar/same as L-N for the supply.

because on most typical TNC-S systems the N and E parts are the same conductor

 

[Lucien Nunes]

Sure, the currents might divide in the ratio you mention. But the effects on the RCDs upstream vs. shower won't be related to that ratio because the current upstream is residual, but in the shower branch not residual. Or have I missed your point?

 

[Eire79]

Out of interest was there anything particular in this property that led to this decision such as measuring DC leakage, or particular equipment being present or is it mainly a preventative measure now dangers of type AC devices has come to light?
I'm reflecting that I'm not sure I'd know when this a must and not just a good idea.
I'm also thinking that BPG4 is 6 years old now and probably overdue a revision to mention type AC RCDs.

I had inspection by inspector, in Ireland I must add, and our new rules say that type AC are not recommended so he said change out devices. I'm of the opinion that yes I should change the main RCD protecting socket circuits but will not on showers as risk of DC LEAKAGE EFFECTS are not applicable. Why waste money to change it. Not recommended and forbidden are vastly different in my book.

 

[Eire79]

It's a bit of a complex thing really, if you supply say a car charging itself, if the electronics fail they can inject a DC back into the system. So say it injects just 10A back into the network.

This current would flow into anything and everything connected - so say you have a Ze of 0.35 ohm (assuming the same resistance for phase-N) and a 45A shower connected (shower would be about 5 ohm) - the DC would flow into these two in parallel - simple maths shows it would feed 10A into ~0.33 ohm (.35 & 5 in parallel) they would share this between them, the grid taking around 9.4A and the shower 0.6A - An AC RCD at the supply intake would be blocked, the RCD on the charger would be blocked (if it's not suitable) and quite possibly the RCD on the shower ( if 0.6A is enough to block it).

In this case the effect would be much wider than the circuit itself!

If the DC fed back is higher then more loads would have their RCD blocked - unloaded and light loaded RCDs wouldn't, only the adjacent circuits which are heavily loaded would!

That is why car charging points have to be protected against DC back feed, and lots of issues Re. suitable RCD for them.

This is all ignored by the silly "granny chargers" - they have no such protection, would be plugged into a std RFC with say type AC RCD/RCBO - so any child coming into contact on that same circuit - or potentially other circuits would not have any protection by the RCD.


For the majority of modern loads however, such as laptops and other AC-DC converters there is no appreciable back feed - just sufficient modification of the ac waveform to block the circuit they are on.

Would you then use only type A and recommend same on all devices just to mitigate against a possible fault occurrence. Change out any type AC you come across.

 

[Julie.]

Would you then use only type A and recommend same on all devices just to mitigate against a possible fault occurrence. Change out any type AC you come across.

I never recommend AC at all these days, but other than for big DC currents associated with vehicle charging I don't think it goes beyond the circuit under consideration enough to worry about.

Wouldn't change them unless there was a specific concern.

 

[Eire79]

I never recommend AC at all these days, but other than for big DC currents associated with vehicle charging I don't think it goes beyond the circuit under consideration enough to worry about.

Wouldn't change them unless there was a specific concern.

Or even the fact that a charger does not exist at present in the premises would allow if I required by me or client the reason not to change them out. And if charger is installed by someone else then the changing of devices becomes a issue for them to change.

 

[Julie.]

Sure, the currents might divide in the ratio you mention. But the effects on the RCDs upstream vs. shower won't be related to that ratio because the current upstream is residual, but in the shower branch not residual. Or have I missed your point?

Agree, however in some older AC types the DC doesn't cancel out - if it is a straight forward single toroidal coil the dc in with be cancelled out by the dc back through and all is as you highlight. Some however use separate magnetic cores and the cancelling out is done on the secondary side via the wiring - this was especially the case for big MCCBs etc.
In this latter case the cores would end up saturated by the DC and the AC would not do anything in the presence of DC.

I don't think the more modern ones use separate cores now, even the largest MCCB have a single core, however the older versions can still be found.

 

[Julie.]

Or even the fact that a charger does not exist at present in the premises would allow if I required by me or client the reason not to change them out. And if charger is installed by someone else then the changing of devices becomes a issue for them to change.

Yes but the circuits that provide sockets etc if there are lots of DC type loads (chargers/laptops etc) it may be prudent, but I wouldn't go with a blanket change no.

 

[Lucien Nunes]

Some however use separate magnetic cores and the cancelling out is done on the secondary side via the wiring - this was especially the case for big MCCBs etc.

OK yes the configuration with separate CT's and balance transformer is a problem with circuit currents containing DC and could be blinded here. I've not seen that type of device in a domestic CU so would tend to discount it for the purposes of the OP's situation, but I'm not saying they don't exist.

 

[Julie.]

OK yes the configuration with separate CT's and balance transformer is a problem with circuit currents containing DC and could be blinded here. I've not seen that type of device in a domestic CU so would tend to discount it for the purposes of the OP's situation, but I'm not saying they don't exist.

I agree I realistically picked a poor example, in hindsight I should of stuck to an industrial case - and they haven't been car chargers but other industrial equipment.


Translated it to domestic without really thinking!