Neutral isolation for TT supply

[Gardner]

Where is neutral isolation required for a TT supply? The way I read it, I need to have isolation on each individual MCB, or does my main DP incomer take care of this?


Isolation of the neutral conductor - Electrical Installation Guide

 

[Deleted member 26818]

Re: Neurtral isolation for TT supply

It's needed wherever isolation for maintenance or safety is required.
Boiler, cooker, extractor fan, etc.
Single pole RCBOs, are acceptable, as long as there is up front double pole isolation.
If there are no socket circuits, or locations containing baths or showers, then an up front double pole 100mA RCD with single pole MCBs, would comply. Wouldn't want an up front 30mA RCD in a house.

 

[Risteard]

Re: Neurtral isolation for TT supply

Boiler, cooker, extractor fan, etc.

All of that could be dealt with by means of the main switch.

Wouldn't want an up front 30mA RCD in a house.

In reality it's unlikely to be a problem unless there is a fault.

 

[Deleted member 9648]

Re: Neurtral isolation for TT supply

It's needed wherever isolation for maintenance or safety is required.
Boiler, cooker, extractor fan, etc.
Single pole RCBOs, are acceptable, as long as there is up front double pole isolation.
If there are no socket circuits, or locations containing baths or showers, then an up front double pole 100mA RCD with single pole MCBs, would comply. Wouldn't want an up front 30mA RCD in a house.

It wouldn't really comply, it would fall foul of the requirements for minimal disruption in the event of a fault on one circuit.

 

[IzzyS]

Re: Neurtral isolation for TT supply

It wouldn't really comply, it would fall foul of the requirements for minimal disruption in the event of a fault on one circuit.

You could argue that the disruption has been minimised - you are not inconveniencing any of the neighbouring installations. What happens when the service fuse fails, or the overhead lines get taken out by a falling tree, or the single up-front time delayed 100mA RCD protecting a TT installation trips?

Should we design in emergency lighting, UPS and/or generator backup and all RCBO protection?

As far as I'm aware, there is no black and white regulation that states a single front end RCD is unacceptable, although the "alternative" regulation writers, such as NICEIC / ECA have their own views.

How does the ubiquitous dual RCD board minimise inconvenience anyway? You still lose half your circuits on the rare occasion an RCD trips.

Such an approach may marginally reduce inconvenience, but it certainly doesn't minimise it.

 

[Risteard]

Completely agree IzzyS (not short for Izzy Stradlin lol?)

 

[DPG]

Of course a dual RCD unit minimises inconvenience. Half the sockets are still on, half the lights are still on. The boiler could well be still on, as could the cooker and outside lights, burglar alarm, who knows. One single up-front 30mA RCD and a bit of extra earth leakage and EVERYTHING is gone. Daz

 

[Deleted member 26818]

Re: Neurtral isolation for TT supply

All of that could be dealt with by means of the main switch.

A main switch is an acceptable method for providing double pole isolation, but it doesn't help if local isolation is required.
For instance, if a gas engineer were to turn up for a boiler safety check, the house holder may prefer for their fridge, freezer, etc. to be left switched on.
Another instance would be an extractor fan in a bathroom.
It may be preferable to remove the cover for cleaning with the lights on?

In reality it's unlikely to be a problem unless there is a fault.

My opinion, is that an up front 30mA RCD will be more likely to suffer nuisance tripping due to cumulative earth leakage from various circuits.
Of course now that 100mA RCBOs are more readily available, the ideal solution is not to use an up front RCD at all.

 

[Risteard]

Re: Neurtral isolation for TT supply

My opinion, is that an up front 30mA RCD will be more likely to suffer nuisance tripping due to cumulative earth leakage from various circuits.

More likely perhaps, but in a typical domestic installation still pretty unlikely.

 

[Gardner]

Re: Neurtral isolation for TT supply

All of that could be dealt with by means of the main switch.



In reality it's unlikely to be a problem unless there is a fault.

Correct, but I can not see someone opening only the main switch to repair a light fitting?

 

[Deleted member 9648]

Re: Neurtral isolation for TT supply

You could argue that the disruption has been minimised - you are not inconveniencing any of the neighbouring installations. What happens when the service fuse fails, or the overhead lines get taken out by a falling tree, or the single up-front time delayed 100mA RCD protecting a TT installation trips?

Should we design in emergency lighting, UPS and/or generator backup and all RCBO protection?

As far as I'm aware, there is no black and white regulation that states a single front end RCD is unacceptable, although the "alternative" regulation writers, such as NICEIC / ECA have their own views.

How does the ubiquitous dual RCD board minimise inconvenience anyway? You still lose half your circuits on the rare occasion an RCD trips.

Such an approach may marginally reduce inconvenience, but it certainly doesn't minimise it.

So why bother with duel RCD boards?....why not fit a 30ma RCD up front on all installs requiring 30ma RCD protection?

 

[Englishman]

Re: Neurtral isolation for TT supply

As far as I'm aware, there is no black and white regulation that states a single front end RCD is unacceptable, although the "alternative" regulation writers, such as NICEIC / ECA have their own views.

I actually put one in for a customer a few months back, he was a landlord who wanted RCD protection but done as cheaply as possibly as he was selling up. It's not the ideal thing to do, but I left the installation in a safer condition that is found it, and I'd rather I do it safely than somebody else hodges it dangerously.

 

[Risteard]

Re: Neurtral isolation for TT supply

I actually put one in for a customer a few months back, he was a landlord who wanted RCD protection but done as cheaply as possibly as he was selling up. It's not the ideal thing to do, but I left the installation in a safer condition that is found it, and I'd rather I do it safely than somebody else hodges it dangerously.

I put one in recently as well for a TT system with no RCD protection! Putting in a 100mA S-type wasn't really an option given that the electric shower; sockets, etc. all would have been left without 30mA protection. As the DB was a discontinued Contactum one (and spares no longer available after contacting the manufacturer) there was really little choice but to mount a frontend 30mA externally (other than replace the DB of course).

 

[Gardner]

So I must ask... how are single pole MCB consumer units legal on a TT supply?

 

[Risteard]

So I must ask... how are single pole MCB consumer units legal on a TT supply?

Because the circuit breaker is not an isolator in this instance - the main switch is.

Even for domestic TN systems all-pole isolation is required, so there is no reason to make a distinction between them and TT systems.