Installing a Doorbell Transformer in a CU

[kevnurse]

Hi Guys,
I've seen a few doorbell transformers being advertised by Friedland and Byron (reputable manufacturers) that can be fitted directly onto the din rail in the CU. This means that, by design, there is no circuit protection in the primary coils of the transformer. I know that the output is usually only 8VDC/1A and I believe the DC circuit returns to the transformer. Would I be right in believing that the lack of any earth or neutral conductor in the transformer assembly means that it cannot possibly short to earth and therefore doesn't need protection?

Also, would I be right in believing that it will not overheat because the only current in the unit will be in the secondary coil only while the doorbell button is pushed?

Thanks in advance for any answers.

 

[Steve]

I believe these Tx's are meant to be on a 6amp MCB. The fact they are DIN rail mountable doesn't mean they are busbar mountable too If you are fitting it in a standard CU, you'd have to cut the busbar short. Personally, i'd fit a second enclosure.

 

[kai]

A din mount doorbell transformer MUST always be fed off an 6 amp or less MCB (Some CU makers do 3 amp ones). Set up a 6 amp or less MCB breaker, and run the red wire to the primary input of the transformer, and the black wire from other primary terminal to the neutral block respectively. Connect the secondary elv side to the bell unit, using two core MAINS rated flex, as the connections will be in the consumer unit adjacent mains voltage circuits. and there you have it.

 

[bernardgreen]

There is some value in earthing one side of the transformers low voltage output. Reason I suggest this is that a fault in the transformer may result in a short between the mains input and the output. ( overheating and the insulation failing and the windings shorting out ).

Some have a interwinding safety screen connected to the DIN rail mounting but in most CUs the DIN rail is not earthed.

I have found a non working door bell circuit that was live due to the transformer partially melting inside.

Modern ones are far better but there is still a perceivable risk

Bernard
Sharnbrook

 

[Steve]

Connect the secondary elv side to the bell unit, using two core MAINS rated flex, as the connections will be in the consumer unit adjacent mains voltage circuits. and there you have it.

I suspect this is the reason DIN-rail-able TX's are usually not din-railed. Its obviously cheaper and easier to use cheap bell-wire all the way! We have 4 bell circuits at work, run in the cheapest nastiest bell-wire ever! 2 of them no longer work. surprise surprise.

 

[kevnurse]

I've now fitted the transformer in the CU at the end of the DIN rail. I sawed off the end of the busbar below the adjacent mcb. The busbar is now held in position by all the mcbs. and isn't going anywhere. I wired the primary side to a 6A mcb shared with my utility lights circuit. OK, its not discriminatory, but neither circuit is that important and there is no more space in the CU anyway; thanks to the transformer. The secondary (bell) wire is sleeved inside orange extension cable outer sheathing from behind the board right up to the point where the 2 wires have to separate to enter their respective terminals. I'm confident that it is the best that can be done without buying a bigger CU.

 

[securespark]

The busbar is now held in position by all the mcbs. and isn't going anywhere.

I'm a liitle worried...What was holding the busbar before you hacked the end off??

 

[Steve]

As I said, you could have bought a seperate enclosure . . .

What happens now when you need another circuit?

 

[bernardgreen]

As I said, you could have bought a seperate enclosure . . .

What happens now when you need another circuit?

Or when the transformer needs replacing.

Good quality transformers have a none re-setable "fuse" in them which "blows" if the windings get too hot. When that happens the transformer is scrap and has to be replaced.

So in a separate enclosure it is a DIY job to replace, in the CU it is an electricians job.

 

[Steve]

So in a separate enclosure it is a DIY job to replace, in the CU it is an electricians job.

Hmm . . . scratches head . . . . He fitted it himself anyway, so whats different about swapping it for a new one?

And Im not sure in what respect you refer to DIY here, but I doubt that opening a CU just to replace a bell transformer is Part P'able. Like for like, and all that. Perhaps a "higher skilled" DIY job.

 

[kevnurse]

Before I fitted the Tx my CU had 4 extra places available at the LH end (furthest from the main switch). The Tx (double mcb size) now occupies the 2 places furthest from the main switch in the CU, leaving 2 places available to the right of the TX. I said previously that I cut the busbar below the adjacent mcb, for brevity. To be exact, I cut the busbar immediately to the left of the spare tooth adjacent to the Tx, thereby completely isolating the TX from the busbar, but leaving 2 places for future mcbs.

The question was asked what was holding the busbar in place before I hacked the end off. Its a Proteus CU (I know, they're not the most popular make.....but I bought the house, not the CU. The CU happened to be come as part of the deal. How do you tell the wife that the house purchase is off because the fuse box is 2nd rate?).

Anyway back to the question: The busbar is held in position at the RH end by its most RH tooth being clamped in the main switch live terminal. At the other end, it is lightly attached to the CU body moulding by a small self tapping screw; but its only screwed there to stop the busbar rattling around when the CU is being carried about prior to installation. The busbar, when in use, is pre-dominantly held in place by the combined terminations of the main switch and 8 mcbs. The loss of the far left end of the busbar is irrelevant to its security.

 

[kai]

Where is the mandatory RCD protecting the downstairs ring final circuit then?

 

[RF Lighting]

Where is the mandatory RCD protecting the downstairs ring final circuit then?

Which regulation says downstairs ring final circuits must be RCD protected?