RCBO's on TT

[Andy™]

PME is mostly used as a cost cutting way of providing an earth by the DNO. we all know of its faults, and there are times that i have TT'd a PME supply, even if it hasnt been getting exported.

these days, just about everything has to be RCD'd, so not really much of a problem with earth loop by sticking in a rod, where as to 16th ed, you would normally have to use a 100mA TD incomer

There is a few times in the past i have made a dual split from a single split when supply is TT and they dont want one RCD on everything, much easier now that dual splits are just about all you can get

 

[steptoe]

I am sorry steps but you are going to have to state reg numbers that say you cannot use single pole Rcbo's on a TT system in a domestic enviroment. If you look in the on site guide on page 16 It has a picture of a TT system which has a consumer unit full of Rcbo's. On my last assesment and before the job I did for the assement I asked if I could use single pole Rcbo's on a TT system in domestic and was told I could. If this was wrong I would like to Know because I do not like doing incorrect work. I understand what you are saying about a neutral conductor being capable of being 240v but if there is a fault surely the Rcbo will disconnect the live conductor and therefore there will be 0 volts at the neutral then. I cannot understand why a TT system should be any different to any other system of earthing ie if you have a disconnected neutral. You have got me worried with this so I am going to have to read the red book through.Batty

dont take this as a precedent,

431.2.1

the neutral conductor shall be protected against short-circuit current.

afaik overcurrent DETECTION and PROTECTION are TWO different things,.

you have to PROTECT the neutral, doesnt mean you have to have detection on it.

remember tho, as I often say, 7671 is NOT law, do what you want, just make sure it is safe, and as far as I am concerned, unless you disconnect the neutral on a TT system then IMHO you will have a code 1.

 

[Andy™]

you have to PROTECT the neutral, doesnt mean you have to have detection on it.

but if the live is switched off, then in effect so is neutral, so no need for that to be switched.

unless there is something badly wrong where the live from one circuit uses the neutral of another

but, 531.2.1 states an RCD should be capable of disconnecting all line conductors at the same time... there is no mention of RCBO's allowed to be SP, so can we assume that SP RCBO's dont comply?

 

[steptoe]

but if the live is switched off, then in effect so is neutral, so no need for that to be switched.unless there is something badly wrong where the live from one circuit uses the neutral of another

but, 531.2.1 states an RCD should be capable of disconnecting all line conductors at the same time... there is no mention of RCBO's allowed to be SP, so can we assume that SP RCBO's dont comply?

right, sorry, but Im still fed up of basic stuff, even in 15th the neutral was considered to be live, IT CARRIES 230V so it is live, if you still dont believe me then please feel free to do the disconnected neutral test.

we really really need to be considering ourselves as diyers or sparks if we cannot grasp the simple fact of what cables carry 230v or not under normal circumstances.

 

[SPECIAL LOCATION]

but if the live is switched off, then in effect so is neutral, so no need for that to be switched.unless there is something badly wrong where the live from one circuit uses the neutral of another

but, 531.2.1 states an RCD should be capable of disconnecting all line conductors at the same time... there is no mention of RCBO's allowed to be SP, so can we assume that SP RCBO's dont comply?

That regulation is saying that IF the RCD is double pole or if its three phase

it MUST be capable to disconnect all of the conductors poles at as damn near simultaneously as possible.

It is NOT saying they have to be double pole!

 

[SPECIAL LOCATION]

Ok heres a bit of bumff wot the NICEIC say:-

A means for automatic disconnection of supply, such as a fuse or a contact of an overcurrent circuit-breaker or RCD, is to be inserted in the line conductor of a single-phase circuit and in every line conductor of a multi-phase circuit. The fuse or contact must be arranged to open in the event of an earth fault affecting the line conductor concerned, in accordance with the requirements of BS 7671 for automatic disconnection (see item 6).

There is no general requirement in BS 7671 for means to be provided for automatic disconnection of a neutral conductor. However, there are a number of exceptions. Examples of where a circuit-breaker or RCD must be of the multipole type which disconnects all line conductors and the neutral conductor are as follows:

The overcurrent protective device of each final circuit in a caravan or motor caravan (Regulation 721.43.1 refers).

Where selected as a main switch for operation by ordinary persons for a single-phase supply, such as in a household or similar installation (Regulation 537.1.4 refers).

Where selected as a means of isolation for a circuit in an installation forming part of a TT system (Regulation 537.2.1.1 refers).

Where selected as a means of isolation for a circuit in an installation in a potentially explosive atmosphere (in order to avoid the possibility of incendive sparking), clause 8.2 of BS EN 60079-14 refers.

Any circuit-breaker or RCD which is inserted in an earthed neutral conductor must be of the linked type, arranged to break all the related line conductors (Regulation 132.14.2 refers).

A single-pole fuse, circuit-breaker or RCD is not to be inserted in a neutral conductor, but only in the line conductor (Regulation 132.14.1 refers).

It is generally sufficient for overcurrent protective devices and RCDs to be of the single-pole type (inserted in line conductors). However, there are exceptions,

Examples of where a circuit-breaker or RCD must be of the multipole type disconnecting all line conductors are as follows:

An RCD used in a circuit having more than one line conductor (Regulation 531.2.1 refers).

In a circuit supplying a multi-phase motor (for example, three-phase), if necessary in order to avoid

 

[Andy™]

right, sorry, but Im still fed up of basic stuff, even in 15th the neutral was considered to be live, IT CARRIES 230V so it is live, if you still dont believe me then please feel free to do the disconnected neutral test.we really really need to be considering ourselves as diyers or sparks if we cannot grasp the simple fact of what cables carry 230v or not under normal circumstances.

yes, the neutral is live when used, but if there isnt any current flowing because the circuit is switched off, then it should be dead. If its a TNCS, then it should be at earth voltage, if TNS or TT it may vary a little more though

the disconnected neutral test has nothing to do with it being switched or not - agreed if you disconnect it you will get mains voltage there, but thats only because the live hasnt been turned off. turn that off and it will drop to 0 (or there abouts)

 

[steptoe]

if you have 2 or more SP RCBOs and turn 1 of them off then all the neutrals will be carrying 230v potential to earth with a disconnected neutral.

 

[Andy™]

if you have 2 or more SP RCBOs and turn 1 of them off then all the neutrals will be carrying 230v potential to earth with a disconnected neutral.

dont really know what your trying to get at....

you get the exact same problem if you had an MCB or fuse thats single pole...

 

[steptoe]

but you wouldnt have a SP MCB protecting a TT installation without RCD protection, the RCD provides the DP protection level.

the point is that on TT you MUST have DP protection on all circuits, even on the HI INTEGRITY side of the board(without a DP RCD present)

 

[Andy™]

easier to just stick a dual split rather than RCBO's