RCD requirements for TT?

[HappyHippyDad]

Evening all...

I'm having a complete mind blank on what the mA requirement is for an RCD on domestic household with TT earthing, Ra = 57ohms. I'm not talking about final circuits.

I'm planning on splitting the meter tails into a henly, then into a switched fuse, then 50m SWA to consumer unit in garage which will contain a 30mA RCD or RCBO's.

I simply cannot remember or find in the regs if I need RCD protection for the buried SWA. If it was a TN then It wouldn't need it as it has mechanical protection, but it's TT.

I'm sure a 100mA S type would be OK in the switch fuse as replacement for main switch, but I just want to know where in regs it says this will comply, or even necessary?

 

[HappyHippyDad]

Thanks for all the input so far everyone.

Table 51.3 BBB p.156 shows me exactly what I need, just couldn't find it! I've got my 18th exam in a few days so I better be able to read a book by then!

Just for your information @pc1966 it will be feeding an EVCP 32A, 1 x 16A radial, 1 x 20A radial and 1 x 6A lights.

 

[Ian1981]

Just make sure it’s a type A rccb if the EV has type A rcbo , if the EV has a type B then the upfront rccb will also need to be type B.

 

[mattg4321]

Just make sure it’s a type A rccb if the EV has type A rcbo , if the EV has a type B then the upfront rccb will also need to be type B.

Just been through this on the other thread.

Can you point out to me where this is stated or what regulation would be contravened.

Not having a go, just want to be sure what is/isn’t allowed.

If I have a EV charger with type B RCD to be fitted to a dual rcd consumer unit fitted only a couple of years ago with type AC RCCB’s then there is no way I can connect to this consumer unit at all if it has no unprotected ways? I can’t find anything in the regulations that precludes doing this though.

 

[Ian1981]

Just been through this on the other thread.

Can you point out to me where this is stated or what regulation would be contravened.

Not having a go, just want to be sure what is/isn’t allowed.

If I have a EV charger with type B RCD to be fitted to a dual rcd consumer unit fitted only a couple of years ago with type AC RCCB’s then there is no way I can connect to this consumer unit at all if it has no unprotected ways? I can’t find anything in the regulations that precludes doing this though.

The DC current passing back through the installation which is why the type B is selected can saturate the coil of the type AC, its rcd selection in part 5.
If your selecting a type B because there’s that level of DC then it stands to reason that any rcd upfront in series requires the same consideration.

 

[pc1966]

If your selecting a type B because there’s that level of DC then it stands to reason that any rcd upfront in series requires the same consideration.

I'm not sure that is fully applicable, as if a pure DC fault appears from the EV charging side the type B RCD should disconnect and then any up-stream type AC/A is no longer blinded to any other faults?

Certainly I would not consider a type AC as reasonable any more given practically everything has the possibility of pulsed DC from a fault on the rectified side of things, and type A are available and usually only a couple of quid more. But so long as the EV charger has its own DC-specific protection I would expect you not to need that duplicated (at extreme cost for type B these days).

 

[mattg4321]

The DC current passing back through the installation which is why the type B is selected can saturate the coil of the type AC, its rcd selection in part 5.
If your selecting a type B because there’s that level of DC then it stands to reason that any rcd upfront in series requires the same consideration.

Surely it won't have a chance to saturate the coil of the upstream RCD as the Type B in the unit itself will already have disconnected?

If I were fitting an EV charger at a garage in a large property with TT earthing arrangement I would likely have problems fitting Type B only RCD's. I'd probably need an S type RCD for the distribution circuit to the garage and I've never seen a Type B, time delayed RCD yet? Maybe they do exist though..

I get that it's desirable to have all type B RCDs. Actually it's most desirable to only have 1 RCD in any circuit because of selectivity, but it doesn't mean you're going against regs if you don't do things this way?

 

[i=p/u]

Just for installation methods would you ya require another rcd lads

 

[mattg4321]

I'm not sure that is fully applicable, as if a pure DC fault appears from the EV charging side the type B RCD should disconnect and then any up-stream type AC/A is no longer blinded to any other faults?

Certainly I would not consider a type AC as reasonable any more given practically everything has the possibility of pulsed DC from a fault on the rectified side of things, and type A are available and usually only a couple of quid more. But so long as the EV charger has its own DC-specific protection I would expect you not to need that duplicated (at extreme cost for type B these days).

This is the point I'm making.

If you are coming into an existing installation that has type AC RCD's on a dual board with no unprotected ways and no room to add another board, it seems absurd to have to change the CU that may only be a couple of years old.

In this case it's 2018 MK board. Don't think MK do even type A RCD's?

 

[Risteard]

I'm not sure that is fully applicable, as if a pure DC fault appears from the EV charging side the type B RCD should disconnect and then any up-stream type AC/A is no longer blinded to any other faults?

Certainly I would not consider a type AC as reasonable any more given practically everything has the possibility of pulsed DC from a fault on the rectified side of things, and type A are available and usually only a couple of quid more. But so long as the EV charger has its own DC-specific protection I would expect you not to need that duplicated (at extreme cost for type B these days).

What if the DC fault current was below the tripping threshold for the RCD? It might still be sufficient to saturate the coils of the upstream RCD(s).

 

[Ian1981]

As said what if slightly less than 10mA passes back through the installation, the adds to any existing earth leakage?

 

[pc1966]

What if the DC fault current was below the tripping threshold for the RCD? It might still be sufficient to saturate the coils of the upstream RCD(s).

That would be unlikely for a 100mA upstream of a 30mA B-type, as even the 30mA trip threshold for the down-stream RCD is a lot less than even the AC up-stream test case's coil would be troubled with. After all it has to be sufficiently linear for the peaks of +/-150mA and core satuarion is going to be at higher levels than that.

I think the real risk of RCD being "blinded" is where there is no DC-sensitive RCD at all and you might have hundreds of mA, or even amps of DC flowing, and the AC-sensitive RCD knows nothing about that fault (unless it came on very suddenly, even then a delay RCD would ignore such a start-spike).

 

[Strima]

The DC is present from the charging unit and is not a fault, it's there all the time the unit is turned on and charging. Same as you get from SMP supplies but on a much greater scale.

This DC can stop the RCD detecting a fault on one of the other domestic circuits and not operate hence the reason for changing it to a more suitable type.

 

[mattg4321]

I can't see any regulation that having a 30mA type AC upstream of a type B in an EVSE would contravene.

If/when I come across this whilst carrying out an EICR I don't see how any code can really be given. Perhaps a note, but that would be my personal opinion and not backed up by any regulation.

 

[nicebutdim]

What if the DC fault current was below the tripping threshold for the RCD? It might still be sufficient to saturate the coils of the upstream RCD(s).

You encounter a lot of TT installations and have posted previously about installing an upfront 100mA Type S RCCB to such installations as a matter of course.

If you were to encounter an EV charger fitted with Type B RCBO, on a TT installation, where DC leakage from the charger could affect operation of the upfront Type S RCCB, what course of action would you be inclined to take that wouldn't give rise to potential issues with selectivity?

Is there a Type B solution that incorporates time delay or am I missing something obvious about the operation of different types of RCCB? I'm not so much concerned about what regulations permit as what provides best protection, while minimising any risk of nusiance tripping.



Sorry if this is a stupid question, but I can't get enough of threads that raise issues specific to TT earthing systems.

 

[pc1966]

The DC is present from the charging unit and is not a fault, it's there all the time the unit is turned on and charging. Same as you get from SMP supplies but on a much greater scale.

This DC can stop the RCD detecting a fault on one of the other domestic circuits and not operate hence the reason for changing it to a more suitable type.

But how much?

I would consider anything more than a few mA to be a fault, as one would expect the system (even at 48V or whatever considered "safe") to have insulation resistance of hundreds of kohm to earth.

If the EV charger is OK for a 30mA type B RCD than it would have to be below 15mA normally, or it is in the trip region. And as above, I can't see that sort of level saturating any type A RCD, and not a 100mA (or above) type AC.

If you had it off a 30mA AC RCD/RCBO I would be a bit more concerned

But for playing with some type-AC RCD FCU recently they would trip on the type A waveform OK, just too low current in one direction, and a little slow in the other direction.

So for sure if you could have significant DC then you need type B, but upstream of that I can't see it causing difficulties for any that are going to be selective anyway (so 100mA or above TT incomers).