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?

 

[telectrix]

i f a 100A S will provide disconnection times the should be right.

 

[Flanders]

Think its table 41.5 with an earth loop of 100 ohms or less you can use a 500ma rcd

 

[Strima]

I'd look at using a 300mA to get your earth fault disconnection time.

I also test L-N to ensure the OCPD will operate in case of a dead short, not normally a problem on TT systems but I've had a couple that have been end of line and the PFC has been pretty low.

 

[suffolkspark]

Yes you need upfront rcd as fuses/mcbs wont provide fault protection, must be a time delay so it doesnt trip before downstream rcds, and if using rcbos downstream make sure they also break the neutral.

 

[Ian1981]

100mA S type for fault protection of the distribution circuit.
I don’t know why people bring up the L-N loop test when they wouldn’t dream of testing it if it’s a TN system, unless everyone claims to do the t=K2S2/I2 calculation for every circuit on a TN or TT system.
Just because the earth fault loop impedance is high on a TT , it’s unlikely that the L-N is high as you’d measure this at the origin on a PFC measurement anyway.

 

[i=p/u]

50/0.5=100ohm (') (')

 

[Deleted account]

Forget about the mA of the RCD. Just concentrate on can you achieve disconnection times For each circuit whether distribution or final.

30 mA, 100mA, time delay or not. These are chosen for discrimination and circuit requirements.

 

[Risteard]

it’s unlikely that the L-N is high

If it was you'd have problems with excessive volt drop too!

 

[pc1966]

If it was you'd have problems with excessive volt drop too!

Yes, checking PSCC is important not just for max MCB break capacity, but as it more or less confirms system voltage drop as well.

 

[pc1966]

As folk have said above, you need an up-front RCD to disconnect on any cable faults as the TT earth wont do so one the OCPD.

For selectivity that has to be x3 above the final circuit RCDs and to have some delay. Most common for domestic would be 100mA S-type (just to add confusion on AC/A/B/F/etc nomenclature!) and they ought to be fairly readily available.

With your rod impedance you could use a 300mA (Ra < 167 ohm) or 500mA (Ra < 100 ohm) delay RCDs. But it is unlikely to be needed as:

• A 100mA S-type copes will cope with up to 50mA normal leakage, which is unlikely in any sort of system (short of a massive amount of computers on single phase)
• You are probably not to cascading further RCDs for sub-DBs where you would need to have again a x3 sensitivity factor and longer delays for hard-fault selectivity.

A final concern for selectivity is the downstream RCD or RCBO must also disconnect the faulted circuit's neutral, otherwise a N-E fault (or hard L-E fault on a low impedance load) will still trip the main RCD as they won't stop current imbalance due to the spurious neutral path.

All standard RCD (e.g. dual RCD bard) do this, but many RCBO do not. In the UK it seems you choice for RCBO would be best from the new compact Fusebox, Wylex, or Crabtree RCBO that are neutral-switching DP style.

TL;DR Use 100mA delay up-front, then DP RCBO board (or dual RCD board if you must).

 

[pc1966]

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.

Just read this again, if it is a TT system at the Henley blocks (supply end, rather than making the garage a separate TT system off a TN supply) then there has to be an RCD after the switched-fuse but before the SWA sub-main run.

Otherwise a fault in that sub-main will simply cause the earth rod to sit at ~230V and nothing will stop it!

 

[cliffed]

100mA S type for fault protection of the distribution circuit.
I don’t know why people bring up the L-N loop test when they wouldn’t dream of testing it if it’s a TN system, unless everyone claims to do the t=K2S2/I2 calculation for every circuit on a TN or TT system.
Just because the earth fault loop impedance is high on a TT , it’s unlikely that the L-N is high as you’d measure this at the origin on a PFC measurement anyway.

That’s a good point

 

[cliffed]

Going back to the L-N loop, it becomes a problem when the Zs on that circuit is high & if on a Tncs some just stick a Rcbo on it.
This protects the earth fault scenario but will the mcb protect the short circuit scenario.

 

[pc1966]

Perhaps over-thinking this, but what is the garage CU going to have?

If it is only lights and a few sockets, you could make the supply cable RCD a 30mA instant one and just have a switch and two MCB at the garage. However, if it really is going to be a separate office or workshop then you should be designing for selectivity, and in that case:

• Fused-switch as high as SWA cable rating will allow to get good selectivity with the final circuit MCB action.
• Ideally sub-main fuse 1.6 time smaller than the DNO fuse for selectivity in the L-N big-bang fault situation. So if 100A DNO fuse, then 63A is going to achieve total selectivity with it (or 80A/50A, 60A/40A)
• Delay 100mA RCD at the feed end, and DP RCBOs to feed each of the final circuits.
• If you are limited to smaller feed fuses, like 40A, then try to use MCB/RCBO no larger than 20A
• If it is a workshop then also consider some emergency lights in case power fails for any reason. Being plunged in to darkness with a lathe still spinning and sharp tools all around is not a good situation!

 

[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).

 

[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).

 

[Risteard]

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.

Sorry, was working until 8:30p.m. tonight so only seeing your post now. I haven't come across an RCBO with a Type B RCD characteristic (though they may exist). Any Type B RCD I have fitted has been an RCCB.

If a Type B RCD is needed then any upstream RCD will need to be Type B also. If Type A, then any upstream will need to be at least Type A.

You shouldn't have any issue finding a double pole Type A S-type RCCB - they are widely available.

 

[nicebutdim]

Sorry, was working until 8:30p.m. tonight so only seeing your post now. I haven't come across an RCBO with a Type B RCD characteristic (though they may exist). Any Type B RCD I have fitted has been an RCCB.

If a Type B RCD is needed then any upstream RCD will need to be Type B also. If Type A, then any upstream will need to be at least Type A.

You shouldn't have any issue finding a double pole Type A S-type RCCB - they are widely available.

No worries about the time and I appreciate the response - it was purely hypothetical and came to mind after Type B requirement for certain chargers was introduced in post #17.

 

[pc1966]

Further to this discussion here:

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