RCD or RCBO

[Steed]

Hi,
I have just had a quote from local spark for installation of new 9kW shower unit. He insists that he needs to supply / fit an additional "Shower CU" alongside my existing 12way consumer unit as I require an RCBO which cannot be fitted into my Contactum CU. My issue is that my CU has an 80A RCD covering the installation and plenty of spare capacity. Surely all I need is a 40A MCB to run the circuit. I have a rod (TT) earth which I'm told has 18ohm "loop".Also a 10mm T&E cable upgrade(which I understand the need for). Am I missing something here?

 

[ricicle]

http://www.tlc-direct.co.uk/Main_Index/Consumer_Units_Index/CP_Range_MCB_RCBO/index.html

 

[Steed]

Thanks ricicle. I assume that contactum RCBO will fit straight on the DIN rail.
Will that trip before the main RCD ?

 

[PrenticeBoyofDerry]

Is your CU is currently protected by a 30mA RCD.
If so and you have spare in box, should be okay with MCB but it maybe a split board partly protected by RCD.
Then a RCBO can be used, which are available.

 

[PrenticeBoyofDerry]

Will that trip before the main RCD ?

There is no point putting an RCBO in RCD protected busbar.
It will not offer discrimination you only need a MCB if it is RCD protected.

 

[flameport]

My issue is that my CU has an 80A RCD covering the installation and plenty of spare capacity.

What type of RCD? 30mA or 100mA or 100mA time delayed would be likely options, although others do exist.

If a 30mA, then as above, no point in fitting an RCBO. A separate box would be required as you already have the entire installation on a single RCD which does not comply, and adding further loads will make the situation worse (that being the entire lot will go off when there is a fault anywhere).

If 100mA time delayed then a 30mA RCBO could be fitted.

If 100mA and not time delayed, it means whoever fitted it was trying to save money and fitting an RCBO won't comply either, so additional box required.

 

[ericmark]

I note a TT system which could mean a 300ma S type RCD protecting all circuits?

Although you state 80A it does not say at what leakage it will trip. Or if there is a delay (S type). I think many of the replies has assumed 30ma but it could well be 300ma.

There are a number of reasons to fit a RCD. Personnel protection needs 30ma but fire only needs 300ma and to allow for poor earth connection it could be in the amps rather than milliamp range. Often we would fit a 100ma with a TT system.

It is easy to jump to conclusions when reading posts and I think this has happened here.

So assuming a 100ma S type is fitted at the moment then to fit a further 30ma RCD would make sense. 6mm cable Reference Method 100# is rated at 34 amp and in real terms that means 32A or 7.36kW so for 9kW clearly a larger cable is required.

 

[riveralt]

Hi,
I have just had a quote from local spark for installation of new 9kW shower unit. He insists that he needs to supply / fit an additional "Shower CU" alongside my existing 12way consumer unit as I require an RCBO which cannot be fitted into my Contactum CU. My issue is that my CU has an 80A RCD covering the installation and plenty of spare capacity. Surely all I need is a 40A MCB to run the circuit. I have a rod (TT) earth which I'm told has 18ohm "loop".Also a 10mm T&E cable upgrade(which I understand the need for). Am I missing something here?

No you are not missing anything - you just haven't told us everything about the circuit.
At the moment you appear to have one RCD covering all your circuits.
Adding a new shower circuit to the existing installation would contradict BRB BS7671 Reg 314.1.
Every installation shall be divided into circuits as necessary to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCD's due to excessive protective conductor currents produced by equipment in normal operation
etc etc.
Since this is a new installation it must comply with the current regulations and as such the proposed solution by the electrician is the most cost effective way of achieving compliance.

 

[Steed]

Thank you everyone. I'm clear now as to the requirement. My CU has a 30mA RCD covering the busbar. The only issue is that it isn't a split-load CU, thus any tripping will take out the whole house. How often is that going to happen? As I see it , the only way that a 40A RCBO is going to discriminate from the main RCD is in an overload situation.

 

[ebee]

The only way it will discriminate is in either overload and/or short circuit if it is fitted into the consumer unit.
Ie an earth fault will not discriminate.

If its on a separate RCD (RCBO for instance) then earth faults will discriminate.

In an ideal world then each circuit would have its own RCD (say a plain switch for the consumer unit and an RCBO for each circuit).
But that is a bit expensive.

So you might split to two RCDs and arrange the circuits so that with an RCD trip you might lose half the installation but still retain some power or some lighting to all rooms/areas.

You might do that but also have plain switch & separate RCBOs for items such as cooker, shower, boiler & freezer.

Or you might as I do and thousands like me have one RCD (a "front ender") feeding the whole installation - about 30 years old, still works and has never been a problem (yet!).

Let`s start a "I am a Front Ender" Club, we could wear badges! javascript:emoticon('')

RCDs were once uncommon.

When they first became popular they replaced the consumer unit main switch to protect the whole installation, so we had an whole or nothing situation. Occasionally one might be fitted externally to the outgoing connection instead just for say a socket circuit. Thousands perhaps millions were done this way for years.

Then years later the split load came along , sockets & shower on RCD lighting not.

Then eventually dual RCDs (Wrongly often called a "17th Edition" consumer unit)

and a (again wrongly called) "High Integrity 17th edition" - Dual RCDs and a few RCBOs.

Usually a front end RCD is not the whopping problem it is often perceived to be! Although it could be.

The only way to find out why the OPs electrician insists on a separate "shower protection unit" is to ask them. It might be discrimination it might be the 80A mainswitch or both.

 

[riveralt]

Thank you everyone. I'm clear now as to the requirement. My CU has a 30mA RCD covering the busbar. The only issue is that it isn't a split-load CU, thus any tripping will take out the whole house. How often is that going to happen? As I see it , the only way that a 40A RCBO is going to discriminate from the main RCD is in an overload situation.

For goodness sake you're quibling over a few quid.
A small CU with Main Switch, RCD and 40 or 50Amp MCB is £17.16 at Denmans - add a few quid for the the service connector blocks and tails and your done.
No issues over discrimination, no issues over losing the whole installation (or half ) and it complies with the regs.

 

[ebee]

Well the OP wanted to know why it "MUST" be done that way at extra cost.
and we`ve also told him the benefits.
Now you`ve told him the costs (other suppliers might cost more).
He can weigh it all up and probably be happier with his electrician rather than wondering if he was guilding the lilly .

 

[Steed]

Thanks again everyone. A good range of views but all constructive. This is what a good forum should have. I now understand why my sparks has opted for the standalone CU. The separate RCBO will "see" any earth leakage/imbalance only on the shower circuit and thus will trip without affecting lighting and other circuits as the regs state.