High Integrity CSU on TT system.

[Geordie Spark]

If a High Integrity CSU where every circuit is protected by a mix of RCDs & RCBOs is connected to a TT system, is it still necessary to fit a 100mA S front end RCD?

 

[Guest123]

No, if every circuit has additional 30mA protection for whatever reason the need for the front end 100mA S type is negated.

Particular care should be given to the supply tails however, if the CU is metalclad as the EFLI may be too high to operate the suppliers fuse in the event of a fault to the casing of the CU.

In some instances it is safer to fit the above S type RCD to protect against the above scenario.

 

[flukey]

no need

 

[Geordie Spark]

No, if every circuit has additional 30mA protection for whatever reason the need for the front end 100mA S type is negated.

Particular care should be given to the supply tails however, if the CU is metalclad as the EFLI may be too high to operate the suppliers fuse in the event of a fault to the casing of the CU.

In some instances it is safer to fit the above S type RCD to protect against the above scenario.

Many thanks for that Lenny

The CU is a 15way plastic Wylex.

I had an argument with the DNO's bloke who arrived to change the cut out over this. He argued that it needed a 100mA rcd & I argued it didn't.

It's nice to have it confirmed here.

Incidentally, the "old" CU that I replaced was metalclad - with no RCD of any kind anywhere - and 4 MCBs ALL of which were rated at 40 Amps !!

This included protection for the lighting circuit which was wired in 1mm 2 core !!!!

 

[Electro-tech]

So what and where is Isolation to the Board ?

 

[Geordie Spark]

So what and where is Isolation to the Board ?

There is a 100A double pole isolator fitted between the meter & the CU which itself has its own built-in 100A double pole isolator.

 

[Electro-tech]

Interesting read OSG page 23. TT conduit installations..If cables in walls have a earthed metallic covering, earthed steel conduit,30ma RCD's will be required for etc . The rest of the installation needs protecting by a 100ma RCD-' S' type or time delay.

 

[Dunc]

Correct me if I'm wrong but isn't there something in the regs that states that RCD protection on a TT supply must isolate both live conductors or in other words be double pole. This means that RCBOs are not permitted as most are single pole.

 

[Guest123]

Correct me if I'm wrong but isn't there something in the regs that states that RCD protection on a TT supply must isolate both live conductors or in other words be double pole. This means that RCBOs are not permitted as most are single pole.

You are right, an RCD should disconnect both 'live' conductors. However this topic is something I questioned a few years back with my NIC inspector as I was concerned that a single pole RCBO that doesn't disconnect the neutral condcutor is more of a danger on a TT system as the neutral conductor cannot reliably be confirmed and held at or near earth potential.

Now there is nothing specifically precluding the use of a single pole RCBO on a TT system in the regs, indeed 411.3.2.1 mentions only the 'line' conductor with no notes or exceptions for TT systems that I can see. Chapter 53 touches on the use of RCD's and also states that a device for 'isolation' should be multipole but again no reference to TT systems and single/double pole RCBO's.

My NIC inspector also confirmed that a single pole device for ADS was suitable for use on a TT system as the device for isolation (main switch) was double pole and if the line conductor was disconnected there should not be any current flow in the neutral, this does not preclude the presence of fault current from another circuit being introduced to the affected circuit via the still connected neutral conductor.

IMO double pole RCBO's are safer but not mandatory.

 

[Guest123]

Just as an add-on to the above that I forgot to mention, a single pole RCBO will disconnect under N-E fault conditions as it will detect the current imbalance between L&N, BUT THE N-E FAULT COULD STILL REMAIN as the neutral is still connected. Again IMO there should'nt be any current flowing in the circuit as the line is disconnected but as I mentioned previously neutral may not be at or near earth potential.

As I said previously....safer but not mandatory.

 

[egginyourface]

What wold happen if the Gas or water pipes became live due to a fault being introduced from next door?

 

[Geordie Spark]

Just as an add-on to the above that I forgot to mention, a single pole RCBO will disconnect under N-E fault conditions as it will detect the current imbalance between L&N, BUT THE N-E FAULT COULD STILL REMAIN as the neutral is still connected. Again IMO there should'nt be any current flowing in the circuit as the line is disconnected but as I mentioned previously neutral may not be at or near earth potential.

As I said previously....safer but not mandatory.

Nothing is straightforward these days is it?

Maybe I should just spent £30 on a Chint time delay RCD & fit that in place of the isolator.