Help explaining layout of Consumer Unit

alex1980 · 19 Apr 2021

Hello,

I have a Tenby CU as shown below. Out of idle curiosity I wondered if someone could explain the layout of it regarding the switches. I understand everything to the left of the RCD is protected and to the right is not, but...

1.) What is the double "Sub Circuit Switch" for?
2.) Why is one immersion heater switch protected and the other not (one left and one right of RCD)

Thanks

davelx · 19 Apr 2021

1) I suspect the Sub Circuit feeds the storage heaters and immersion only
2) I suspect that the Sub Circuits are NOT RCD protected, only the three circuits between the RCD Switch and the Sub Circuit Switch.

However, it's impossible to be sure without removing the cover of the CU and having a look at how it's wired up.

AndyPRK · 19 Apr 2021

1 to switch off in summer I guess ?

alex1980 · 19 Apr 2021

@AndyPRK, good point, thanks. I don't have storage heaters anyway hence why I switch them off.

plugwash · 19 Apr 2021

It's impossible to be 100% sure from the pictures alone but I suspect it's a CU with two seperate incoming feeds.

A permanently live feed feeds the main switch on the right, which in turn feeds the right hand bank of MCBs and feeds the RCD in the middle. The RCD in turn supplies the middle bank of MCBs.

The right hand switch and bank of MCBs are probablly fed from a totally seperate incoming feed. This would have originally been fed from a switched output on the meter (of it's associated time clock), but it may have been transferred over to a permanent feed if the dual-rate meter was removed.

Off-peak electric heating/hot water setups often have two immersion heaters, one is located near the top of the tank and is fed via a switch from the permanent supply. This heater is intended to be used when hot water is needed quickly. The second heater is located near the bottom of the tank, this is fed from the off-peak supply and is intended to operate every night to heat up the whole tank using cheap electricity.

Do you have a picture of the wires coming into your CU and/or a picture of the service head and meter? if so that may help clarify the situation.

Also (unless there are other labels that are out of shot), I'm really not impressed with the labeling by whoever installed your CU, anything out of the ordinary and *paricularly* CUs with multiple separate incoming feeds should be clearly labelled.

Sir-John · 19 Apr 2021

davelx said:
1) I suspect the Sub Circuit feeds the storage heaters and immersion only
2) I suspect that the Sub Circuits are NOT RCD protected, only the three circuits between the RCD Switch and the Sub Circuit Switch.

However, it's impossible to be sure without removing the cover of the CU and having a look at how it's wired up.

You suspect correctly, It's a Dual Tarrif board.

JohnD · 19 Apr 2021

plugwash said:
It's impossible to be 100% sure from the pictures alone but I suspect it's a CU with two seperate incoming feeds.

I'm not at all sure of that, because the "Main Switch" appears to turn everything off. And is a 2-pole switch so cannot be switching two feeds.

otherwise, both the Main Switch and the Sub Circuit Switch are incorrectly labelled (which is not impossible)

Sir-John · 19 Apr 2021

As you have no storage heaters, have the points been converted to 13A sockets? If so then maybe the consumer unit has been doctored to reflect this.

alex1980 · 19 Apr 2021

@Sir-John, no I recently updated all sockets\switches as the old ones were all tatty, but just kept like for like. I don't need extra sockets, and if I ever sell the flat the switches are there for anyone that wants to put storage heaters back in.

SUNRAY · 19 Apr 2021

plugwash said:
It's impossible to be 100% sure from the pictures alone but I suspect it's a CU with two seperate incoming feeds.

In which case it's wired incorrectly. "Main Switch" should isolate the whole unit [apart from the incoming tails]. If it only isolates part of the unit it should be labelled differently.

Sir-John · 19 Apr 2021

Ive only ever seen dual taraff boards with two Main Switches BUT labled as such. No different to having two cu for whole installation. On peak and off peak.
As pugwash said can we have a picture of the meter area including tails etc.

alex1980 · 19 Apr 2021

Few extra images of internal wiring and meter area...

SUNRAY · 19 Apr 2021

Sir-John said:
Ive only ever seen dual taraff boards with two Main Switches BUT labled as such. No different to having two cu for whole installation. On peak and off peak.

I'd have no issue with either format but one device labelled 'Main Switch' which only isolates part of the CU is very wrong.

Sir-John · 19 Apr 2021

As it's all on a single Tarrif (24hr) it should never have been left as a dual set-up. I agree it wants the consumer unit correcting to reflect this and 2nd (LH) switch rewired or removed.

AndyPRK · 19 Apr 2021

Why fix WHATS not broken.

At least the tails are there if they are required in future

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Help explaining layout of Consumer Unit

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