what size wiring- elecrtic shower

[JohnW2]

It may be possible to install an RCBO of the required size on the non-RCD side of that board. This is only a solution for TN supply types. It does not work for TT as the RCD would have to be double pole.

What makes you say that?

Kind Regards, John.

 

[Risteard]

It may be possible to install an RCBO of the required size on the non-RCD side of that board. This is only a solution for TN supply types. It does not work for TT as the RCD would have to be double pole.

What makes you say that?

Kind Regards, John.

Indeed. In a domestic premises isolation is achieved by means of the main switch. It is a nonsense to suggest that all protective devices must be double pole.

 

[JohnW2]

Indeed. In a domestic premises isolation is achieved by means of the main switch. It is a nonsense to suggest that all protective devices must be double pole.

That's certainly my interpretation of the regs - which themselves only require that items of equipment have 'a' means of DP isolation in TT installations - however, as you say, that can be a main switch in a CU. I think the confusion (alternative interpretations) comes from the OSG - I can't remember the exact wording, but I think it can just about be read as suggesting that each item of equipment in a TT installation should have its own (perhaps 'local') method of DP isolation.

Kind Regards, John.

 

[RF Lighting]

The only time you would need to use DP RCBOs on a TT supply would be if you are providing 30mA protection to individual circuits, and there is an upfront 100mA TD main switch.

A N>E fault on a final circuit would trip a single pole RCBO,as you would expect, but would still be visible to the 100mA TD RCD and trip that too.

 

[securespark]

naked wet bodies

I've had to go and lie down

So did I, but then I had to quickly get up again because all I could think of lying down was

naked wet bodies

But then I got dizzy again getting up too quickly and had to lie down once more, thus prompting another rapid rise from the bed....

It took a good slapping from Mrs Secure to snap me out of this vicious circle...

 

[echoes]

Indeed. In a domestic premises isolation is achieved by means of the main switch. It is a nonsense to suggest that all protective devices must be double pole.

That's certainly my interpretation of the regs - which themselves only require that items of equipment have 'a' means of DP isolation in TT installations - however, as you say, that can be a main switch in a CU. I think the confusion (alternative interpretations) comes from the OSG - I can't remember the exact wording, but I think it can just about be read as suggesting that each item of equipment in a TT installation should have its own (perhaps 'local') method of DP isolation.

Kind Regards, John.

The OSG says that DP isolation must be used for a TT system where isolation is required that does not interrupt the supply to other circuits. Which implies that a SP RCBO cannot be regarded as a means of isolation in a TT installation, but as a means of protection. The examples of TT CU's clearly show SP RCBOs (but no up-front 100mA RCD).

 

[JohnW2]

The only time you would need to use DP RCBOs on a TT supply would be if you are providing 30mA protection to individual circuits, and there is an upfront 100mA TD main switch.
A N>E fault on a final circuit would trip a single pole RCBO,as you would expect, but would still be visible to the 100mA TD RCD and trip that too.

Yes, if one wanted to have 'discrimination' in that situation, one would need DP RCBOs. However, do people generally do that - and, if so, how easily found (and how expensive) are DP RCBOs? I have to say that, apart from a 30mA B40 Contactum one which one sees quite often 'for showers', they are not something I normally see at all (not that I look for them) - and I suspect that (like the Contactum one), those which exist are probably two modules wide.

I have to say that such a situation does exist in my TT installation, since I have a few (SP) RCBOs and have retained the 100mA TD RCDs as 'isolators' (and 'belt & braces' RCD protection), even though all final circuits have RCD or RCBO protection. Whilst some might say that theoretically is non-compliant with 314.1, there are separate 100mA TD RCDs on each of the three phases, and I have cross-phase (cross-floor) emergency lighting which automatically activates in the case of failure of one phase (and power failure alarms on freezers) so I'm actually pretty comfortable about the 'division of the installation'.

Kind Regards, John.

 

[JohnW2]

That's certainly my interpretation of the regs - which themselves only require that items of equipment have 'a' means of DP isolation in TT installations - however, as you say, that can be a main switch in a CU. I think the confusion (alternative interpretations) comes from the OSG - I can't remember the exact wording, but I think it can just about be read as suggesting that each item of equipment in a TT installation should have its own (perhaps 'local') method of DP isolation.

The OSG says that DP isolation must be used for a TT system where isolation is required that does not interrupt the supply to other circuits. Which implies that a SP RCBO cannot be regarded as a means of isolation in a TT installation, but as a means of protection. The examples of TT CU's clearly show SP RCBOs (but no up-front 100mA RCD).

You are confirming the difference between the OSG and BS7671 which I mentioned in the text you quoted. 5.1.1(ii) of OSG does, indeed, talk about separate DP isolation for "every circuit or group of circuit which may have to be isolated without interrupting other circuits", but I'm not sure in what circumstances that emboldened words apply, or where they come from. The OSG cites 537.2.1.1 of the regs, which merely state that there should be a means of DP isolation for every circuit (in a TT installation), but without any mention of any circumstances in which the isolation device should not affect other circuits (apart from the generality of 314.1).

The absence of an up-front 100mA RCD obviously removes the 'total shutdown' issue to which RF lighting refers, but (as you imply) still leaves several SP RCBOs on a common main switch (as illustrated in RCBO) without facilities for separate DP isolation - so the whole situation is a bit confused/confusing.

Kind Regards, John.

 

[westie101]

Looking at the wording, it does not seem to specifically state that the DP isolation should be by a device, just that there should be a means of DP isolation.
I would suggest that a SP device and physically disconnecting the neutral would be a means of DP isolation!

 

[JohnW2]

Looking at the wording, it does not seem to specifically state that the DP isolation should be by a device, just that there should be a means of DP isolation. I would suggest that a SP device and physically disconnecting the neutral would be a means of DP isolation!

Hmmmm - that seems a bit like suggesting a pair of boltcutters with insulated handles (and other appropriate PPE!) as being 'a means of isolation' - perhaps true on your side of the meter, but I'm not sure that BS7671 would see it that way!

However, to be serious, as I've said, BS7671 itself only seems to require a means of DP isolation (e.g.the main switch) - it's the OSG which tries to go further and complicate things a bit.

Kind Regards, John.

 

[westie101]

Interesting concept as we have a nationally agreed definition of isolation in the Electricity at Work Regulations, from Regulation 12

(2) In paragraph (1), “isolation” means the disconnection and separation of the electrical equipment from every source of electrical energy in such a way that this disconnection and separation is secure.

There is no mention of any device at all, so why should BS7671 have a different view?

 

[EFLImpudence]

Yet more anomalies ?

In the definitions 'isolation' is ...

separating the installation or section from every source of electrical energy.

Yet table 53.2 lists numerous single pole devices as capable of isolation.


Also 537.2.1.1 states...

it is not necessary to isolate or switch...

My point being (as Westie wrote) perhaps disconnecting a conductor IS regarded as an acceptable method?

 

[westie101]

Disconnecting a conductor is an accepted method of isolation (by a minimum gap of 9inches at 11kV)

It's also worth pointing out that on our "side of the meter" we are not required to disconnect the neutral whilst isolating in any circumstances.

 

[JohnW2]

Interesting concept as we have a nationally agreed definition of isolation in the Electricity at Work Regulations, from Regulation 12

(2) In paragraph (1), “isolation” means the disconnection and separation of the electrical equipment from every source of electrical energy in such a way that this disconnection and separation is secure.

Yep, BS7671 seems to subscribe to that nationally-agreed definition. (apart from having changed the words and word order a bit!). It defines "isolation" as:

A function intended to cut off for reasons of safety the supply from all, or a discrete section, of the installation by separating the installation or section from every source of electrical energy"

There is no mention of any device at all, so why should BS7671 have a different view?

Ah, well, you see, BS7671 then goes on to define "Isolator" as:

"A mechanical switching device which, in the open position, complies with the requirements specified for the isolation function. An isolator is otherwise known as a disconnector"

Does EAWR have no equivalent of that, or does it leave you free to use the boltcutters 'for the isolation function'?!

Kind Regards, John

 

[JohnW2]

Yet more anomalies ? In the definitions 'isolation' is ... separating the installation or section from every source of electrical energy. Yet table 53.2 lists numerous single pole devices as capable of isolation.

Yep, seemingly more anomolies - unless they want to get into a serious 'quibble discussion' about the meaning of 'electrical energy'!!

Also 537.2.1.1 states... it is not necessary to isolate or switch... My point being (as Westie wrote) perhaps disconnecting a conductor IS regarded as an acceptable method?

True - but, in addition to 'common sense' (if one can talk of that in relation to the regs!), all subsections of 537.2.2 are about 'Devices for isolation' - albeit 537.2.2.4 talks about neutral links that can be removed with a tool (mighty close to 'disconnecting a conductor').

Kind Regards, John.