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?

 

[pc1966]

Is there a Type B solution that incorporates time delay or am I missing something obvious about the operation of different types of RCCB?

Not that I have seen. Doepke have a good range of type B (at a price!) but none are delay ones to mk knowledge.

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

No, it is a perfectly sensible question to ask!

 

[Ian1981]

Types of rcd in series
Images courtesy of BEAMA

 

[Ian1981]

Some EV manufacturers like Garo particularly stipulate this requirement, that a type AC cannot be upfront of their type A etc.
If also rcd manufacturers are telling you the same thing then that’s good enough for me.

 

[nicebutdim]

Types of rcd in series

That states that no Type S RCCB should be fitted upstream of a Type B or F, which obviously removes any issue of selectivity.

Would this be appropriate for a TT earthing system, given that disconnection may rely on electronics at a single point?

 

[Ian1981]

That states that no time delayed RCCB should be fitted upstream of a Type B or F, which obviously removes any issue of selectivity.

Would this be appropriate for a TT earthing system, given that disconnection may rely on electronics at a single point?

Not sure I follow?

 

[HappyHippyDad]

Types of rcd in series
Images courtesy of BEAMA

In your picture Ian, it shows a type A RCD upstream from a type B RCD. It then says a type A RCD should not be upstream from a type B RCD. I'm a little confused?

edit... No it doesn't! My error.

Although It does look like it is suggesting no upfront S type RCD if there is a type B RCD downstream (as there are no S type type B RCD's)

 

[nicebutdim]

It doesn’t, the paragraph above is for S types, the one below is for general rcd selection

My mistake, I was fixated by the diagram.

What I've been driving at is the fact that this discussion has highlighted that operation of Type S RCCBs may be impacted by DC earth leakage from devices that require protection of a Type B RCBO. As such I'm wondering if an upfront Type S RCCB could be rendered ineffective in such circumstances and, if so, what the most appropriate solution might be.

 

[HappyHippyDad]

My mistake, I was fixated by the diagram.

What I've been driving at is the fact that this discussion has highlighted that operation of Type S RCCBs may be impacted by DC earth leakage from devices that require protection of a Type B RCBO. As such I'm wondering if an upfront Type S RCCB could be rendered ineffective in such circumstances and, if so, what the most appropriate solution might be.

Higher rated type B RCD. E.g 300mA Type B? I can't imagine fitting one though!

 

[nicebutdim]

Higher rated type B RCD. E.g 300mA Type B? I can't imagine fitting one though!

I could stand to be corrected, but I believe that this wouldn't appropriately address the issue of selectivity as sufficient earth leakage could cause disconnection at either point.

 

[mattg4321]

So if you want to fit an EVSE on a TT supply with upfront S type you effectively can’t?

 

[Ian1981]

Yes you can, the S type must be A type if the EV has an A type in it.

 

[nicebutdim]

So if you want to fit an EVSE on a TT supply with upfront S type you effectively can’t?

That's the conclusion I'm arriving at, if the charger requires Type B RCBO, and why I'd directed the question to Risteard. I figure he might have considered this possibility.

Edit: Just spotted @Ian1981 post, which seems to confirm this thought.

There doesn't yet appear to be a great selection of available 2 pole time delayed 100mA Type S RCCBs, with Type A characterisitcs and I'm surprised that Wylex haven't yet introduced a version of the widely used WRMT100/2. Currently the best priced and most widely available options appear to be those from CP Fusebox and Lewden.

 

[nicebutdim]

Not that I have seen. Doepke have a good range of type B (at a price!) but none are delay ones to mk knowledge.

As best I can ascertain, they manufacture a couple of industrial Type B time delayed RCCBs, but nothing suitable for a typical domestic installation.

DFS4 B and DFL8 B

 

[Strima]

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).

From my understanding A type are up to 6mA and F&B types are up to 10mA. Without knowing the manufacturers information I couldn't comment on how much leakage they have but some do stipulate B type RCDs so it must be a fairly high current they generate during normal use.

 

[Risteard]

So if you want to fit an EVSE on a TT supply with upfront S type you effectively can’t?

You can. I did two recently for a well-known major courier company.

One was single phase and I replaced the Type AC S-type with a 300mA Type A S-type, along with a Type A 30mA for the EVSE - which had inbuilt 6mA DC leakage protection.

The other was three-phase so I replaced the Type AC with a four pole 300mA Type A S-type, and a four pole Type A 30mA for the EVSE (again with built-in 6mA DC leakage protection).