Bell Transformer

[peterduckett]

First of all I'm no spark and i completely understand that work on the consumer unit in this was my abide to Part P but i have two questions on fitting a Bell Transformer

1) I know you have to provide over current protection for the transformer but what about Earth leakage?

I have been thinking about this and not sure it would make sense as surely the RCBO would only protect the 230v cable which would wholly be in the consumer unit and any break on the 12v side would not cause the RCD to trip.

Is my thinking correct?

2) These Transformer say that you can wire the 12v in bell wire unless it is to be mounted in the consumer unit, in which case you must supply protection to the 12v against 230v

How do people do this? Sleeving? and i guess this is only for the amount of cable within the consumer unit

 

[ban-all-sheds]

1) No RCD protection is needed, and none is required by the regulations. If you bury the 12V cable at less than 50mm depth etc (see IEE Article on 17th Edition requirements for concealed cables.) then you have to provide RCD protection for that, which is a little tricky to do - I think they must have written that part of the regs on a Friday afternoon.


2) Just use cable rated at 230V for the output of the transformer.

 

[peterduckett]

Thanks BAS but if you created a short in the 12v side would it trip and RCD on the 230v side I'm not convinced it would.

Surly as the two side are not physically connected in the transformer all that would happen is that more current would flow on the 12v side until the protecting device reaches its maximum 3 or 6 amp.

Any suggestion on cable i would preform as small as possible.

 

[tonyelectric]

yes you are right the transformer is an isolating transformer meaning that the two coils are separate, not overlapping so if one breaks down then there is no possibility of mains voltage presenting on the 12V side.

What BAS is referring to (I think) is the concept of using a 12V RCD on the secondary - no such product exists to my knowledge, the output should have a fast blow fuse within the tranny.

The problem with using 230V rated cables on the secondary side is they are often too big to fit in the terminals of bells and pushes, I would oversleeve the ends of the bell wire with sleeving rated for 230V like the blue and brown stuff available for oversleeving red & black - don't use earth sleeving to avoid confusion with earths

 

[ban-all-sheds]

Thanks BAS but if you created a short in the 12v side would it trip and RCD on the 230v side I'm not convinced it would.

No it wouldn't.

What BAS is referring to (I think) is the concept of using a 12V RCD on the secondary - no such product exists to my knowledge

I've never heard of one, but on that fateful Friday afternoon the Wiring Regulations committee forgot to exempt ELV cables from 533.6.7 and 522.6.8....

You just have to pretend that they did. Does anyone know if that's going to be fixed in the Amendment?

The problem with using 230V rated cables on the secondary side is they are often too big to fit in the terminals of bells and pushes,

But probably not too big for the output terminals of DIN rail mount transformers.

I would oversleeve the ends of the bell wire with sleeving rated for 230V like the blue and brown stuff available for oversleeving red & black - don't use earth sleeving to avoid confusion with earths

Is that rated at 230V - i.e. is it sold to be used as insulation, or just for identification?

I'd use 1mm² twin & earth to a junction box outside the CU, then bellwire from there.

 

[bernardgreen]

I would fit the transformer in a single ( or double ) module outside the CU and provide 230 via a switched fused outlet.

Three reasons why I would not fit it in the CU

[1] If the transfomer fails then the CU has to isolated until it is repaired. Some of these transformers are not designed for continuous load so a jammed door bell push could lead to an over heated transformer.

[2] There is small risk that the magnetic field from a transfomer built to a low specification could affect an adjacent MCB's tripping point.

[3] There is a small risk that an over loaded or failling transformer could become hot ( I have seen one scorched ) and affect adjacent MCBs

I also earth one side of the 12 volt so that in the event of the transformer failing and 230 volt coming in contact with the 12 volt circuit there will be a trip of the RCB due to current from the 230 live reaching earth.

I was called to a house many years ago after a caller got a shock from the door bell push button. Inside the transformer there a short between live and the centre of the secondary winding.

 

[ban-all-sheds]

I also earth one side of the 12 volt

Mustn't do that if it's a SELV transformer....

 

[bernardgreen]

I also earth one side of the 12 volt

Mustn't do that if it's a SELV transformer....

Why not ? It may not be essential to earth but you seem to be suggesting it is banned.

 

[ban-all-sheds]

It'll be designed to be separated. Un-separating it will be installing it incorrectly.

 

[PrinceofDarkness]

Bernard: Have you any idea what 'SELV' means?

I suspect not, so I'll tell you that a 'Separated' ELV output has no possibility of a connection to Earth - that's what it's all about. So why the Dickens are you going on about connecting a SELV circuit to Earth?

I also find your cockeyed reasoning about modular transformers within a CU, quite riduclous: Magnetic fields interfering with adjacent MCBs indeed!

"You couldn't make it up" (unless your name were 'bernard'!)........


Lucia.

 

[bernardgreen]

Bernard: Have you any idea what 'SELV' means?

Yes.....

I suspect not, so I'll tell you that a 'Separated' ELV output has no possibility of a connection to Earth -

so there is no possibilty of a fault that might affect the SELV circuit

that's what it's all about. So why the Dickens are you going on about connecting a SELV circuit to Earth?

In the event of a fault ( they do happen ) one side of the SELV circuit may come in contact with a 230 volt phase conductor. ( melted wires due to a loose MCB terminal in a CU with built in bell transformer )

I also find your cockeyed reasoning about modular transformers within a CU, quite riduclous: Magnetic fields interfering with adjacent MCBs indeed!

I sem to recall it was mentioned on this forum as being a possibility

"You couldn't make it up" (unless your name were 'bernard'!)........

What is you opinion of this "made up comment"

Assume a TT installation where the CPC doe not go to earth but goes to the coil of a voltage operated earth leakage breaker. Earth faults raise the CPC to the voltage that trips the breaker.

Now consider an appliance with a metal case connected to the CPC is placed on a metal surface that is earthed via metal pipe work to true earth. The case of that appliance has created an alternative route from the CPC to true earth and effectively shorted out the coil of the earth leakage breaker. The earth leakage breaker is therefore in-effective or far less sensitive to earth "faults" than it should be.

Now was that a reason to introduce double insulated items that MUST NOT BE EARTHED even if they have metal cases. IN fact they must not be connected to the CPC of a TT system in case the case accidently creates a circuit that bypasses the earth leakage breaker coil.

 

[flameport]

In the event of a fault ( they do happen ) one side of the SELV circuit may come in contact with a 230 volt phase conductor.

The definition of SELV excludes this possibility.
414.4 is very specific about how SELV circuits and items supplied from them are installed, one key point being that the wiring must be physically separated from other circuits, or have additional insulation.
Running single insulated 'bell wire' or similar into a consumer unit is not permitted.

If a fault causes 230v to appear on SELV wiring, then it wasn't installed in accordance with BS7671.
Connecting one side of it to earth won't help either, as the size of the SELV wiring would probably be too small to carry the fault current.

Assume a TT installation where the CPC doe not go to earth but goes to the coil of a voltage operated earth leakage breaker

Such installations have been obsolete for the best part of 30 years, primarily for the very reasons you mentioned.

 

[tonyelectric]

The reason double insulated appliances oftem bear the legend MUST NOT BE EARTHED has developed for two reasons, but has nothing to do with your theory that the case could by-pass an RCD, the same would be true of a single insulated earthed appliance.

The two reasons are:
Take a new Anglepoise lamp, it's all metal but with a double insulated lampholder and two core flex, the legend must not be earthed, is applied to prevent joe public from connecting either the live or neutral to earth and causing havoc - if you felt so inclined to re-wire with 3 core cable and earth the frame it would still be safe, and identical to previous generations of the same item.

Now take a DVD player linked to a CRT TV, both are DI, put your AVO between the case of the DVD player and a good earth, typically 70-110V is detected but with hardly any current (created by induction around the screened components), now connect an earth to the same DVD player and link to an RCD protected socket, there is a chance of nuisance tripping - back in 2002 when I worked for Comet's service department this was a genuine reason for product return because people were receiving minor shocks (like static shocks) between their DVD/VCR etc when leaning on a radiator to put in a tape - so some DIYers had a go at earthing their kit which resulted in nuisance tripping

In fact in the early days of DI earths were routinely still fitted, I have a 1970 Wolf Safetymaster DI drill, the internal construction is DI but they still earthed the metal shell "just in case"

 

[ericmark]

Oh what a tangled web we weave! It seems in amendment one the mistake will be removed. I had not realised how bad of a mistake it was until I read about earthing a SELV supply.

The problem I see with any type tested distribution board (Consumer unit) is the manufacture has to authorise the use of any item fitted inside the board. Hence why even if they fit we can't use another make of MCB or RCBO.

So a bell transformer made by Friedland can't be put inside a MK consumer unit unless you have written permission to do so. However items designed to fit inside enclosures often do not require at tool to access the low voltage terminals and also have cooling slots too big to comply with IP limits i.e. 1mm from top.

Much easier to use a transformer designed to be fitted outside the distribution unit. The cable can be very thin and I used alarm cable to back door push button and repeater speaker. But there are of course many makes with many different power ratings and I used an old Gents transformer inside my power supply for my radio and it can supply 10A without a problem. Many of the burnt out transformers are due to miss matching. The clock outlet does nicely to supply a transformer fused to 1A and if push does get stuck in easy to unplug. Yet not able to be used by others so not unplugged to use drill etc.

 

[tonyelectric]

This is especially daft as quite a few brands of MCB are the same product made under different badges (like the Electrium stuff), and what do you do if you have an obscure branding that is some wholesalers "own brand" on what is obviously a Wylex box for example. Heres an idea, if we scraped off the branding from MCBs & boxes and put our own company names on customers would be stuck with us for the life of the board!!!!