Quick question - Reg number prohibiting SP RCBOs on TT

[Adam_151]

Struggling to find one as long as you have an all pole isolator present.

It goes against the grain, but I can't seem to find the reg. Anyone able to point me in right direction.

Cheers

 

[Risteard]

Struggling to find one as long as you have an all pole isolator present.

It goes against the grain, but I can't seem to find the reg. Anyone able to point me in right direction.

Cheers

There isn't one unless you are making a claim on them for isolation purposes. The main switch will provide all pole isolation.

 

[ericmark]

531.4.1 If an installation which is part of a TT system is protected by a single RCD, this shall be placed at the origin of the installation unless the part of the installation between the origin and the device complies with the requirements for protection by the use of Class II equipment or equivalent insulation (Section 412). Where there is more than one origin this requirement applies to each origin.
Table 53.2
(5) In an installation forming part of a TT or IT system, isolation requires disconnection of all the live conductors.
See Regulation 537.2.2.1.
531.2.9 Where, for compliance with the requirements of the regulations for fault protection or otherwise to prevent danger, two or more RCDs are in series, and where discrimination in their operation is necessary to prevent danger. the characteristics of the devices shall be such that the intended discrimination is achieved.
NOTE: In such cases the downstream RCD may need to disconnect all live conductors.
532.1 Where, in accordance with the requirements of Regulation 422.3.9, it is necessary to limit consequence of fault currents in a wiring system from the point of view of fire risk, the circuit shall be either:
(i) protected by an RCD complying with Regulation 531.2 for fault protection, and
- the RCD shall be installed at the origin of the circuit to be protected, and
- the RCD shall switch all live conductors. and
- the rated residual operating current of the RCD shall not exceed 300 mA
or (ii) continuously monitored by an insulation monitoring device(s) complying with Regulation 538.1 and which
initiates an alarm on the occurrence of an insulation fault.
NOTE 1: A fault location system Complying with Regulation 538.2 which is able to locate the faulty circuit may be helpful,
NOTE 2: For locations liming a risk of explosion see BS FN 60079-10. BS EN 60079-14, BS EN 61241-10 and BS EN 61241-14
NOTE 3: The following methods which are additional to the requirements may also be employed:
- devices intended to provide protection from the effects of are faults
- devices intended to provide protection in case of overheating
- optically operated devices that provide signalling to another device intended to break the circuit
- smoke detection devices that provide signalling to another device intended to break the circuit.

I would agree rather vague and it would be hard to say it does not comply where a all pole isolator feeds the unit. There are many places where technically the installation may not comply. HSE rules about live working means the main isolator needs to be in a separate enclosure to the rest of the circuits so we can isolate the consumer unit before removing the cover elsewhere yet we all know this was rare, although the DNO have in some areas installed their own isolator.

The main point is:-
537.2.1.1 Every circuit shall be capable of being isolated from each of the live supply conductors. In a TN-S or TN-C-S system, it is not necessary to isolate or switch the neutral conductor where it is regarded as being reliable connected to Earth by a suitably low impedance.
Provision may be made for isolation of a group of circuits by a common means. if the service conditions allow this.

So rather than the regulations saying you must switch neutral with TT what it says you don't need to with a TN hence being hard to find.

However this would mean even before we had RCD protection all the MCB's on a TT system would need to be double pole. In Europe this is common but rare in the UK. So to try and say one needs double pole RCD's would also mean we need double pole MCB's and if you can find a consumer unit in the UK which will do this you are doing well.

So I would for a TT system use a 100ma RCD to feed consumer unit and then 30ma RCBO's in the consumer unit or use a pair of 30ma RCD's in the consumer unit. I don't say this complies but we have to face facts everyone is doing it this way and to try and follow the regs to the letter just does not work.

The same applies to yellow flex with 55-0-55 supplies we know the core colours should be brown, black, and green/yellow but we can't buy that so we use brown, blue, and green/yellow like everyone else.

 

[Risteard]

I oversleeve the blue brown for RLV.

Although I did have to laugh after purchasing a 110V extension lead where it had an important notice on it that the blue must be used for neutral. Clearly whoever stuck the sticker on didn't realise that there is no neutral conductor in a Reduced Low Voltage system - simply two line conductors which should both be brown.

 

[JohnD]

since you are generally not required to have RCBOs anyway, I thought that 537.2.1.1 meant you have to have a DP main switch (which I'm sure you have)

If you choose to add SP RCBOs as well, they are surely irrelevant to 537.2.1.1, because that is not their purpose.

Doesn't that make sense?

If you have a 100mA RCD as well because it's TT, the RCD has got to be time delayed otherwise it will (probably) trip every time one of the RCBOs doesi

 

[securespark]

Thought there was a 0.2 disconnection time on TT?

Which makes it a bit tricky to comply?

 

[JohnW2]

Struggling to find one as long as you have an all pole isolator present.
It goes against the grain, but I can't seem to find the reg. Anyone able to point me in right direction.
Cheers

As others have said, I don't think there is such a regulation, unless you were trying to rely on the RCBO for isolation (which you wouldn't usually be, in the presence of a main switch).

The main problem of SP RCBOs with TT systems is if there is an upstream (100mA Type S or whatever) RCD. A N-E fault will trip the RCBO, but will not clear the fault, so the up-front RCD will then also operate. If such an up-front RCD is present, you therefore could argue that SP RCBOs were a problem in relation to 314.1(i)

Kind Regards, John.

 

[Tech99]

The same applies to yellow flex with 55-0-55 supplies we know the core colours should be brown, black, and green/yellow

Strictly speaking according the wiring regs, they should be brown, brown, and green/yellow, as it's a single-phase circuit.

 

[Risteard]

The same applies to yellow flex with 55-0-55 supplies we know the core colours should be brown, black, and green/yellow

Strictly speaking according the wiring regs, they should be brown, brown, and green/yellow, as it's a single-phase circuit.

Agreed.

 

[JohnW2]

The same applies to yellow flex with 55-0-55 supplies we know the core colours should be brown, black, and green/yellow

Strictly speaking according the wiring regs, they should be brown, brown, and green/yellow, as it's a single-phase circuit.

Agreed.

I think we've debated this one before but, as far as I am concerned, 50-0-50 (I presume eric meant with 180 degree phase difference) is as much two-phase as supplies with any other phase angle between conductors - and Fig. 3.3 of 312.1.1 of the BGB seems to agree with me. However, I'm not sure whether the fact that it's essentially a one-phase circuit actually effects the theoretically required conductor colour identification.

Kind Regards, John.

 

[Tech99]

The 110V site supplies in the UK can be derived from either a centre-tap earth 110V transformer, in which case it's a single-phase supply, or from a 3-phase secondary with star-point earth, in which case the supply is obviously three phase.

But in either case, when you make a two wire connection to get 110V for a single-phase power tool, the circuit concerned can only be single phase.

The table in the regs specifies brown for the "phase of a single-phase circuit." Black is listed only for "phase 2 of a three-phase a.c. circuit." So whichever of the two types of supply is being used, for a single-phase circuit both conductors should be brown. Not that it really matters that much, from the practical standpoint.

 

[JohnW2]

The 110V site supplies in the UK can be derived from either a centre-tap earth 110V transformer, in which case it's a single-phase supply, or from a 3-phase secondary with star-point earth, in which case the supply is obviously three phase.

That's the point we've debated before, and opinions clearly vary. As I said, I (and the current regs) would call the former a two-phase supply. Indeed, to those of you who do not agree with that, if you don't regard that arrangement as two-phase, what would you regard as two phase? What makes you feel that the centre tap of such a supply differs conceptually from the star point of a 3-phase one?

But in either case, when you make a two wire connection to get 110V for a single-phase power tool, the circuit concerned can only be single phase. The table in the regs specifies brown for the "phase of a single-phase circuit." Black is listed only for "phase 2 of a three-phase a.c. circuit.".

Yes, as I almost got as far as conceding in my previous message, I think I have to agree with that. Indeed, as you say, the same would probably be true if one took a 2-wire 400V circuit from a 3-phase supply. Whether that is necessary sensible is another matter - particularly in the latter case (3-phase supply), whilst it wouldn't normally matter, there are times when it would be desirable to know which of the two phases were present in the 2-wire circuit.

Kind Regards, John.

 

[ricicle]

It's all in the terminology. The supply is derived from two phases but the equipment is only single phase as there can only be one phase current

 

[JohnW2]

It's all in the terminology. The supply is derived from two phases but the equipment is only single phase as there can only be one phase current

I think we're now all agreed about that.

And yes, it's all just a matter of terminology. However, the debate that has been re-opened is about the terminology for supplies, not circuits. I (and the BGB) believe that a supply derived from a centre-tapped transformer is 2-phase, whereas Tech99 (and plenty of others in the past) appear to regard it as a single-phase supply.

Kind Regards, John.

 

[ericmark]

Oh dear I was not trying to take over this post with the phase colours of either a "Split Phase" or "Three Phase" supply all I was doing was pointing out it's not blue.

The debate on wording has come up before with people pointing out there is no such thing as a 2 phase supply. Not sure I would agree with that where only 2 phases from a 3 phase supply are used as in Algiers back in 1980 when I worked there I would call it a 2 phase supply. But where it's not got from a three phase transformer but from a single phase transformer then I would think the Split Phase terminology better describes it.

But does it really matter? And this is my main point. Be it a SELV supply which it said needed RCD or earthed outer protection when buried in a wall neither would do any good. Or the 3 meter rules from protective device which would mean no spur is allowed from a final ring over 3 meters long we need to look at it and use some common sense. This quoting regulations is all very good but likely there will be an exception to every rule.

However back to orignal question. The fact that supplies to boats and caravans and a few others need double pole. See 740.537.2.2 - 721.537.2.1.1 - 709.537.2.1.1 - 708.553.1.13 - 705.422.7 - 702.55.4 - 551.7.2 - 531.2.9 then one would think all others could use single pole why else would it be so carefully listed as a requirement for boats and caravans? JohnW2 did point out with a 100ma RCD feeding a problem in relation to 314.1(i) combined with 531.2.9 I would say where there is a main incoming RCD then all RCBO's would need to be all pole to comply. However my point is do we really need to stick to the regulations so rigidly? Specially since with best will in the world we would find it hard to buy a consumer unit which would do the job. Not sure about using distribution units as not type tested for domestic use so it would comply with one set of regs then fall foul of another.

I think common sense should prevail.