How do I add a bell transformer to THIS consumer unit

I'm trying to add in a bell transformer to my existing Hager consumer unit. It's from about 1995. I live in England. The first image below shows the front cover, with five spare ways and their covers. So far so good.



But the next image shows the existing MCBs that are fitted. You can see that there is a gap between them. You can also see that there is no traditional metal DIN rail. This is something a bit like it made of plastic that I'm going to call the DIN plate. It's screwed into the same position that the DIN rail would have gone. However, you will see from this image and the subsequent one that the DIN plate is in fact Tee shaped. It is not a purely horizontal rail. There is a vertical leg in the middle of it. This leg fills the gap between the existing MCBs either side.




You can see therefore that there is no way to attach other items to the middle section of the DIN plate. The consumer unit is effectively full, even though there is a gap and provision for other items in the cover. You can also see that the bus bars allow for other items.

The final image is a smaller Hager unit next to this one, also featuring such a DIN plate. It too has spare terminals along the bus bar, and removable covers in the enclosure.



I've talked to an electrical supplier, and he's never heard of such a thing. I've looked on Google Images for a 12 way (6 + 6) Hager consumer unit, and they all have the traditional metal top hat DIN rail. There is not a single similar consumer unit anywhere on Google.

Why do I have something that Google doesn't? And more importantly, how can I fit my bell transformer?

It was probably a cheaper way of manufacturing it.

As for adding a bell transformer - you can't. Buy and fit an external one.

flameport said:

you can't

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Are you sure? Looking at the detail images ( nos. 3 and 4), it seems that the DIN plate is much wider that a standard 35mm DIN rail. It appears that the existing RCDs are actually just hooked over the top of the DIN plate, and fixed by the busbar lugs. They do not appear to be clipped onto the plate at all as the sprung metal clip can be seen halfway up the plate, with the clip's top about level with the opening in the DIN plate. Can they really be attached in this manner? I'll go yank on the MCBs and see if they rotate about the top of the DIN plate.

Incidentally, if I go with an external bell transformer I'll have to wire it into one of the existing MCBs. Probably one of the 6 amp lighting ones as I obviously can't add a new dedicated one.

Incidentally2, if they were saving money, why not just make a smaller consumer unit initially? It seems that the front cover is from a proper 12 way unit, but the insides are maxed out at 7 ways. My house is going to burn down

PaulUszak said:

Looking at the detail images ( nos. 3 and 4), it seems that the DIN plate is much wider that a standard 35mm DIN rail. It appears that the existing RCDs are actually just hooked over the top of the DIN plate, and fixed by the busbar lugs. They do not appear to be clipped onto the plate at all as the sprung metal clip can be seen halfway up the plate, with the clip's top about level with the opening in the DIN plate. Can they really be attached in this manner? I'll go yank on the MCBs and see if they rotate about the top of the DIN plate.

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Albeit quite a long time ago, I've seen CUs like that (can't remember which), with MCBs etc. "just hooked over the top of the DIN plate, and fixed by the busbar lugs".

PaulUszak said:

Incidentally, if I go with an external bell transformer I'll have to wire it into one of the existing MCBs. Probably one of the 6 amp lighting ones as I obviously can't add a new dedicated one. Incidentally2, if they were saving money, why not just make a smaller consumer unit initially? It seems that the front cover is from a proper 12 way unit, but the insides are maxed out at 7 ways.

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I don't fully understand your explanation as to why additional devices could not be "hooked over the top of the DIN plate, and fixed by the busbar lugs" - are you saying that the "Tee shaped" top only exists in the middle section, and that this prevents devices from being 'hooked over'?

... and Incidentally3, in case you don't realise ... Adding a dedicated MCB (if you could) would certainly be 'notifiable work' ("new circuit"), and I imagine that some people would debate whether adding a bell transformer within a CU constituted adding a new (hence notifiable) circuit!

Kind Regards, John

As said the MCB hook on to the raised rail and then held in place by thier terminal being clamped to the bus bar,

The bus bar is fixed to the case


and as such is part of the structual support for the MCBs. Unlike DIN rail CUs where the MCBs hold the bus bar in place

The advice to fit an external transformer is good advice.

Consumer Units are type tested distribution units. To keep it being a consumer unit rather than losing the type testing status you can only fit items recommended by the manufacturer. The manufacturer will only recommend fitting their units so unless Hagar make a bell transformer for that unit then you can't use one and retain the consumer unit status.

As to why we use consumer units I don't know. Somewhere around 1996 we got consumer units and at the time I was not working in the UK. I think it's something to do with covers over live parts so one can work on them without isolating else where. The HSE is insistence with distribution units they have to be isolated else where which in a house would often result in breaking the DNO seal on the fuse. I have not seen anything which says consumer units can be worked on without isolating else where, but other wise can't see what is the difference between a consumer unit and distribution unit. All I know is where there is not a skilled person in charge then it needs to be a type tested distribution unit.

Although having the transformer inside the box is neat it also means you need mains rated cables for the extra low voltage supply. So often this means a junction box next to consumer unit to connect bell wire to mains rated cables, so in real terms may as well fit the transformer outside anyway.

Even if your cu had a different din rail you would still need to clear 2 spaces from the bus bar and manually wire it to a 6a Mcb.

I don't think they are designed for connecting straight to a bus bar.

Never understood why people want them in CUs anyway.

To be tidy.

People don't want big transformers next to their cu.

The Wylex consumer units employed a similar method of fixing the breakers to the busbar, where the breakers hooked over a stepped bar at the top, and the lower terminal was tightened to grip a solidly mounted busbar at the bottom. I don't know do modern Wylex units use this method, but my own Wylex unit from 2010 manufacture certainly does.

The "fixed bus bar" models use that method.

As opposed to the "flexible bus bar" where you can vary how many MCB's you want on each side.

p.s the bus bars aren't flexible! You cut them to length to suit your install.

AndyPRK said:

I don't think they are designed for connecting straight to a bus bar

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The one I was intending on using does. It' a ... (Byron 7770, £13 from B&Q)

You can clearly see the DIN rail profile at the rear, a sprung securing clip at the top and this intention explains it's funny MCBesque geometry. Seems damn stupid to just mount a transformer or MCB by hanging it off the DIN plate. What happens when you remove the bus bar strip at the bottom for alterations? Do all the mounted components simply fall off? You'd never see this in an industrial distribution cabinet.

I don't doubt they physically mount to a din rail. What I was trying to say is that they don't allign with the live bus bar for easy connection.

As to your question. You don't remove the bus bar and if you did Gravity would hold the mcbs in place.

PaulUszak said:

The one I was intending on using does.

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You really should bother to read the instructions.

3) Connect the mains cable to the 220/240V terminals on the bottom of the transformer and screw down to secure

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it does not say anywhere connect to bus bar. There are units designed to fit in consumer units. However as I said before you have to get one designed for your make and model of consumer unit made by the same manufacturer as consumer unit. You can't mix and match.
You can buy a din rail box to mount items in which require hard wiring rather than using a bus bar. The problem with bell transformers is we have 8 volt, 12 volt, and 16 volt plus some are 12 volt DC some of the ding dong type have a really high current 1A or more where those with musical chimes can be very low power. So I would buy transformer matching bell and mount next to consumer unit.