New CU - looking for knowledge

[chrispriddyuk]

Hello all

It's OK, I understand the Part P stuff and the insurance invalidation stuff and the *risk of death* stuff and I am not proposing that I do this job myself.

I am simply interested in learning and understanding coz I find it interesting.
I've done a fair amount of reading in forums and stuff about how the job of replacing a CU is done and I want to know if, they way i would do it, is actually the way the spark will do it when I call him in. I just wana understand what he is proposing when he tells me what he is going to do, plus I kinda want to *be right* by working out the correct way of doing it.

So...
We have an old fuse wire type CU.
It has 3 fuses: 5a Lighting, 30a Downstairs ring and 30a Upstairs ring.
There is no oven wiring as its all gas and there is no electric shower (it runs off the boiler with a pump)

If I was doing the job I think I would put a new insulated consumer unit in (prob a small 5 way one)
I would have a 6a MCB for the lighting, a 32a RCBO for the downstairs ring and a 32a MCB for the upstairs ring.

I guess they would either pull the company fuse and get them to reseal or have them fit a switch first.
I don't think the house is big enough to warrant a split board due to the small number of circuits.

I don't really understand how to know what type of earthing it has (TT......). The lighting circuit is earthed and there are earth wires on the gas meter and the boiler pipes. And I dont understand what differences this makes to the replacement of the consumer unit.

Am I on the right thinking lines or will it be done differently?
Thoughts, comments, more insight.... thanks all.

Chris

 

[333rocky333]

I would say because MOST instalations rely on eebads.

The earthing system has to be in order for the devices in the cu to operate properly.

Therefore the new cu would be not much use if the earthing etc, was not in good order,

A large number of faults are live to earth and the mcb needs to disconnect the fault , so the circuit earthing is essential.

The water and gas is basically Bonded to keep the services at the same potential, rather than to be used as a conductor

TT etc can usually be recognised by where the earth wire is connected , at the main supply cable which is close to the meter.

 

[streetlighter]

Always worth installing a larger unit than you need now as you knever know what the future needs might be.
A 10 way split load with all the MCBs is probably cheaper than buying a small unit with an RCBO and it would also provide RCD protection for the upstairs sockets too.

 

[ban-all-sheds]

I don't really understand how to know what type of earthing it has (TT......). The lighting circuit is earthed and there are earth wires on the gas meter and the boiler pipes. And I dont understand what differences this makes to the replacement of the consumer unit.

If it's TT then the whole installation needs to be RCD protected, not just some of the sockets, and all of the sockets must be on a 30mA RCD.

 

[very_pointy_ears]

Agreed with streetlighter about fitting bigger CU.

I've just replaced my own CU. The old one had 5 fuses and circuits:
- two 30A fuses for the rings,
- a 30A fuse for the oven,
- a 15A fuse for the immersion + boiler
- a 5A fuse for lights.

I installed a 17 way one and have used 14 of them. I now have:

- the two original rings
- a new ring for the kitchen
- a new separate ring just for the fridge and freezer (on own RCBO)
- a feed for the power showers (not electrically heated)
- a new separate ring for the computer room (on own RCBO)
- a new separete feed for the garage (was spurred off one of the rings before)
- I've split the previous lighting circuit into two
- a new dedicated lighting circuit for the kitchen (lots of downlighters)
- a new feed for some mains powered smoke alarms
- a new feed for the shaver socket in the en-suite
- a feed for the immersion heater and boiler
- a feed for the oven

I've got 3 unused ways in case I want to add anything later (eg dedicated hifi feed).

The reason I put the fridge/freezer and computer room on their own RCBOs is that then they're not on the main RCD. That means that if there's a fault with the electrics that trips the RCD then we don't lose the food in the freezer. Same goes with the computer room - I don't want all of my servers going down if something else trips the main RCD. Also, computer power supplies can be leaky and so this protects everything else from them too.

 

[ColJack]

I hope you have some sort of mains faliure alarm on those mains smoke alarms?
if not then a tripped breaker can be fatal..
most people wire them from the hallway or landing lights since this then gives a sign if the breaker trips..

 

[Chri5]

I hope you have some sort of mains faliure alarm on those mains smoke alarms?
if not then a tripped breaker can be fatal..
most people wire them from the hallway or landing lights since this then gives a sign if the breaker trips..

Come on CJ, surely all domestic 240v alarms are spec'd with battery back up- which when deployed due to mains going u/s will given auibal warning of mains not being on.

95% of all the spec's I've reviewed in the last year for loft conv, extensions have clearly indicated this.

Back on topic...........

Consider the electrical works as an important asset to the property since surveys will highlight the condition and suggest appropriate action such as a PIR.

Doing is well is best.

Why not go for the standard:-

kitchen ring
grd ring
1st ring
gr light
1st lights
Loft area service (if ever there's the potential to convert)
Boiler radial
Immersion heater radial
Misc ciruit- understairs light, door bell Xformer

10mm earth bonding of gas and water
25mm tails, 16mm MET

Always try and finish a job with 20% spare capacity (in ways) on the CU

 

[ColJack]

you missed off shower and cooker circuits...
even if it's power shower now, and gas cooker, the capacity ought to be there in case 5 years down the line, you decide to go electric because future gas prices have rocketed due to the ridiculous future tax on fossil fuels and carbon emissions..

 

[bongos]

- a new separate ring just for the fridge and freezer (on own RCBO)

I thought you normally didn't put fridge/freezer on a RCBO but put it on its own dedicated radial? If the main RCD trips then wouldn't the supply for this circuit go off as well anyway? Or is it on the non RCD side of a split board (think i've answered my own question...sorry )

 

[securespark]

- a new separate ring just for the fridge and freezer (on own RCBO)

- a new separate ring for the computer room (on own RCBO)

Oops!

And if the RCBO should trip when you're away (in the case of the F/F) or when you're on the PC, what then?

You don't need RCD protection for these circuits. Just label them "FOR COMPUTER ONLY" and "FOR FRIDGE/ FREEZER ONLY" and wire them through an MCB.

 

[ban-all-sheds]

You don't need RCD protection for these circuits. Just label them "FOR COMPUTER ONLY" and "FOR FRIDGE/ FREEZER ONLY" and wire them through an MCB.

After June you'll almost certainly not be able to avoid it.

 

[ricicle]

You don't need RCD protection for these circuits. Just label them "FOR COMPUTER ONLY" and "FOR FRIDGE/ FREEZER ONLY" and wire them through an MCB.

After June you'll almost certainly not be able to avoid it.

FCUs on their own cct it is then..... (and buried at more than 50mm )

 

[very_pointy_ears]

- a new separate ring just for the fridge and freezer (on own RCBO)

- a new separate ring for the computer room (on own RCBO)

Oops!

And if the RCBO should trip when you're away (in the case of the F/F) or when you're on the PC, what then?

You don't need RCD protection for these circuits. Just label them "FOR COMPUTER ONLY" and "FOR FRIDGE/ FREEZER ONLY" and wire them through an MCB.

I figured that the RCBO would be less likely to trip than the main RCD because it's only supplying two appliances (F/F). And due to the location of the sockets, I can't bet someone won't try to plug an extension lead in and trail it outside.