CU replacement advice needed

[StudWall]

Please can someone give me some guidance on what to expect when getting a spark in to replace the consumer unit.

I live in a small bungalow which has an overhead TT system.

Currently have 3 circuits served by a prehistoric CU with fuse wires (Lights, Sockets & cooker) i believe this is earthed to a water pipe

i also have a single CU with 40a RCBO for the shower, which has its own earth spike in the garden.

i'd like to have the consumer unit upgraded to something more modern with RCD protection for all circuits, move the shower supply into this unit and also have the earth provided by the spike instead of the water pipe.

As far as I was advised my new CU would be arranged as such

Main Switch - DP 30mA RCD - separate MCB's For the 4 circuits

however, when i rung a local electrician, they started saying that i must have a split load board with the upstairs lights and downstairs sockets on one side, and downstairs lights and upstairs sockets on the other immersion heater on one side and frigde on other etc etc

I reiterated at this point that a bungalow might be missing the whole upstairs thing plus there is no immersion heater or separate circuit for the fridge.

He seemed to be unable to comprehend that we have only 4 circuits and still insisted that i must have a split load board, or use separate RCBOs
(which i questioned as RCBOs are generally SP and apparently TT must have neutral disconnection, hence the DP RCD ... he didn't seem happpy about my comments)

i guess what i'm trying to ask is do i really need a split load board for 4 circuits? seems mental to me or is the suggestion of:

Main Switch - DP 30mA RCD - separate MCB's For the 4 circuits

ok?

Oh and if there is any decent electricians in Basingstoke, please let yourself be known

 

[RF Lighting]

If you're having an up front RCD, then there is no need for a main switch as well. The RCD will do this job.

Personally I'd go for an all RCBO installation. As you only have 4 circuits, it's hardly going to break the bank, and leaves you with a better installation.

With a single up front RCD, a fault any where on your installation leaves your entire installation dead. A fault on the lights leaves you with no sockets, no heating etc.

The water pipe should not be relied upon to provide the earth, and a proper electrode should be installed. It's possible that the existing shower electrode is sufficient, but would need the electrician to confirm.

 

[PrenticeBoyofDerry]

Firstly I would expect the electrician to perform a electrical conditional installation report prior to any unit change.
This will offer both the installer and yourself vital information of any upgrades or remedial work that is necessary, to give you a safe and functional swap over.
With regards to the CU and RCD protection it is highly recommend and also covered in BS7671 (wiring regs) that prevention against complete power loss through nuisance trips is avoided. So either a board populated with two RCDs protecting divided circuits, a board populated with RCBOs or a combination of RCD and RCBO. I would suggest you also had a number of spare ways, just in case you required additional circuits at another time.
It possible that the main earth bonding to gas and water pipes would need updating and the earthing conductor and an electrode.
The EICR would identify most issues.
It seems that your electrician is offering some sound advise!
A CU change is deemed notifiable work and this is a legal obligation in England and Wales, so make sure your electrician can self cert or building controls will need notifying prior to work starting, this includes a fee.

 

[JACKC]

As above, in my opinion you are not asking for rewire so insisting on splitting the circuits is not required. I would carry out an electrical installation condition report first and then discuss any faults found. You may decide on a rewire depending on how the testing goes. Say no problems than go for the rcbo set up and leave the circuits as is. I never change a cons unit without testing first. And then make decisions with the customer depending on outcome of testing.
Water pipe earth will need to go, probably an upgrade on bonding, meter tails and main earth size.

 

[StudWall]

Thank you all for your replies.

My initial thoughts were to ask for an all RCBO layout, however as these are SP they do not comply with the reqirements of a TT system to have the neutral disconnected, this is why i thought a DP RCD first would be needed, followed by MCBs, can anyone confirm please?

also you mention meter tails needing replacing as well as the main earth and bonding ... what exactly is bonding?

Lastly could anyone hazard a guess as to an estimate for this? using the RCBO layout (and Wylex prices from screwfix as a good guide) i expected quote such as:

Consumer Unit: £30
4x RCBO : £150
25mm Meter tails and 16mm earth (3m): £30
3 hours labour : £200

= £410

the sparky i called gave me an estimate of double that for his split load board

 

[PrenticeBoyofDerry]

What requirement/information are you referencing to with regards to SP vs DP RCBO/RCD of TT installations and neutral to be disconnected?

I estimate about £180 for board and RCBO

The installation of a suitable earth arrangement and main earth bonding would depend on your circumstance, so would be difficult to hazard a guess.

There is very little point trying to quote/estimate you labour cost, there are so many variables that may apply to you and until you have an EICR done, it would be pure guess work. I personally do not quote for CU swaps until I have performed an EICR.

Tails are the cables supplying the neutral and live from the meter.
With regards to main earth bonding look at this link;

//www.diynot.com/wiki/electrics:earthing_supply_types_and_bonding

 

[LearningSpark]

A board change and all the testing in 3 hrs

 

[StudWall]

Copied from elsewhere:

BS 7671 REG. 537.2.2.1 states " A device for isolation shall isolate ALL live conductors( That means Line and Neutral) from the circuit concerned, subject to the provision of Regulation 537.1.2 which allows Isolation of the LINE only on TNS and TNCS systems

I think 3 hours is reasonable based on there being only 4 circuits and all wires being accessible from the ground or loft, not having to dig up floorboards etc

 

[AndyPRK]

I look forward to the replies on this. opcorn:


If you want to be complient with the neutral disconnection, then fit a dual RCD CU.

You could have sockets & shower on one side and lights & cooker on the other. Therefore always having cooking facilities, but when the RCD goes the kitchen would be in darkness.

Or lights on their own and sockets, cooker, shower on the other side.
No cooking facilities or means of heat when RCD goes.

I think I would just fit all RCBO's.

I believe there is one brand that does single module size, dual pole RCBO's.

or fit dual module size, dual pole RCBO's. If you can get them still ?

 

[mfarrow]

BS 7671 REG. 537.2.2.1 states " A device for isolation shall isolate ALL live conductors( That means Line and Neutral) from the circuit concerned, subject to the provision of Regulation 537.1.2 which allows Isolation of the LINE only on TNS and TNCS systems

Yes it does, that is under section 527.2.2 Devices for Isolation. There is however a difference between isolation and the requirements to remove danger, as provided by an RCBO.

In a TT installation, the RCD or RCD-component-of-an-RCBO serves two purposes (sometimes both):
- To provide automatic disconnection of supply in the event of a failure of insulation in a Class I environment
- To provide additional protection where 'normal' people are using 13A sockets on the installation, or Class II wiring systems are installed <50mm below the surface of partitions.

Providing Automatic Disconnection of Supply

This is generally provided by an RCD in a TT installation because the earth fault current is not enough to rupture a fuse or trip a circuit breaker.

411.3.2.1 states:
"Except as provided by Regulations 411.3.2.5 and 411.3.2.6, a protective device shall automatically
interrupt the supply to the line conductor of a circuit or equipment in the event of a fault of negligible impedance
between the line conductor and an exposed-conductive-part or a protective conductor in the circuit or equipment
within the disconnection time required by Regulation 411.3.2.2, 411.3.2.3 or 411.3.2.4."

The definition of 'Line conductor' does not include the neutral. Therefore no disconnection of neutral is required for this requirement, even in a TT environment.

Providing Additional Protection

Most (I think all in fact) of the requirements for 30mA RCDs, including additional protection for sockets and cables in walls mentioned above, point towards 415.1.1 which reads:

"The use of RCDs with a rated residual operating current (I&#916;n) not exceeding 30 mA and an operating
time not exceeding 40 ms at a residual current of 5 I&#916;n is recognized in a.c. systems as additional protection in the
event of failure of the provision for basic protection and/or the provision for fault protection or carelessness by users."

No mention of it needing to be double pole there either.

My personal hypothesis is that it is generally deemed that disconnecting only the Line conductor does not present any danger if the neutral is still connected and touched, because neutral will be at or below a safe voltage limit compared to earth in most circumstances. Of course, fault conditions, such as failure theft) of the DNO's transformer earthing can cause the neutral voltage to rise considerably, but I assume this has been dismissed in terms of requiring mitigation (for now).

When you isolate a circuit, you are likely to want to work on it. Touching together neutral and earth of a TN installation is fairly low risk - both are connected together at at least one location and the potential difference between them will therefore be quite low. Where a TT supply exists however, touching together earth to neutral will result in a link being generated between two earthing systems, which may be at different potentials up to the safe touch limit, and thereby risk causing injury to the person working on the installation.

The continent of course are ahead of use, and use DP RCBOs quite often. You can of course therefore buy DP RCBOs (hager do a good one which you can use with an existing SP board) but they are rare and expensive in the UK.

 

[bernardgreen]

The definition of 'Line conductor' does not include the neutral. Therefore no disconnection of neutral is required for this requirement, even in a TT environment.~~~~~

~~~~~My personal hypothesis is that it is generally deemed that disconnecting only the Line conductor does not present any danger if the neutral is still connected

Double pole is necessary where ever there is a significant risk of the local network neutral becoming disconnected or worse becoming live by contact with a phase conductor. Open wire over head distribution having the highest risk of that happening. And because supplying a network earth to a property involved the cost of installing and maintaining a third conductor it was more cost effective in open wire distribution to use local earth rods and hence a TT supply configuration in the property.

 

[DaftPunk]

I'm yet to hear of an installation losing the suppliers neutral only in all many years. Even if the neutral were lost, what hazard would it pose on a TT supply?

You'll still have a double pole isolator up front on the installation anyway.

 

[StudWall]

Interesting points made, I guess I was assuming the rcd was classed as an isolator.

So think I'd be best off with the rcbo option.

Also I wonder if there is any regulation on the length of earth wire allowed from CU to my spike? The meter is about 2m from the cu so a 3m tails kit will be fine but I doubt the 3m earth will be enough to get from cu to spike

 

[AndyPRK]

I thought your electrican would be doing this and gettting the parts.
I'm sure they can get tails cheaper than you.

I wouldn't bother changing the tails unless they look in poor condition.

 

[StudWall]

Yes I will be getting an electrician to do the job but I like go have a look around at approximate prices so I can know when I'm being diddled.

Yes the tails will need replacing, it looks like someone used them for acupuncture practice