12v Halogen Transformer

[sbelec]

Good Afternoon,

Would appreciate a few thoughts, and before anyone shouts this is NOT my design or installation a fault find and repair job!

This is the "roof line" flat in a converted mill, all windows are velux style as the roof slopes through most of the flat. There are no lights into the ceiling except in the false ceilings over the hall and bathroom. The CU is on the false wall between the hall and kitchen just below the false ceiling.

6amp mcb with 1.5mm cable into a junction box with 2 sets of din rail terminals.
The nuetrals go into alternate connectors on the left, line goes into right hand terminals.
The right hand terminals have one leg of double brown T&E which goes to light switches and the return from the switches goes into the alternate terminals on the left with the nuetrals.

I therefore have the "switched" live and nuetral for the bedroom (1 light) and lounge/dining/kitchen (3 lights) in the junction box.

Each circuit then feeds a transormer in the false roof void over the hallway.

The load side of each transformer is a 10mm T&E (E not connected) through the wall horizontally to an appropriate plasterboard box where it is terminated in a connector block with a simple lid. By the side of these terminals are the ends of 12v halogen multilight strung wires strung across the rooms.

The 10mm cables would not fit the transformer terminals so 4 strands into the connection and the remainder wound round the bare copper. The bedroom transformer had failed and the first couple of inches of cable heat damaged.

I tested the circuit, no problems. I wired the bedroom cable into one of the other transformer and the lights in the bedroom worked fine - new transformer required.

I purchased a halolight 50 - 150w unit (5 x 20w lamps) and wired this with a 2,5mm cable into terminal and into connector block with the 10mm and the lights did not work.

The halolight says that the maximum length of cable load side is 2m without explanation, I assumed vault drop but with the 10mm portion being 0.18v I was happy design was good.

I chose the 50 - 150w unit as slightley oversized rather than just the 105w unit, as with 5 lamps it comfortably achieved the minimum load of 50w (no dimmers).

At the moment I have assumed a faulty transformer but would ask the collective for a few thoughts before I go in tomorrow and exchange.

There is no way that the transformer can be closer due to the type of wall void, lack of air and availability for fixing. Plus I do not want to get into recabling!

Many thanks

steve

 

[Darkwood]

Not saying you have but if you are making alterations or additions to an existing circuit you should ensure it complies fully to regulation BS7671 if the previous work was substandard you cant use it as an excuse like 'it was like that already im just repairing it'.

That aside for future reference - 12v hal' are becomming old hat now and the best route to go down would have been retrofit GU10 with LED's but obviously this would be the customers call on the cost side. I note the above as you seem to say that you have reduced the 10mm at point of connection due to the fact it wouldn't fit into the terminals -this is poor workmanship at the very least and you should have foreseen this situe' .... exactly how are the existing fittings connected to this 10mm its a little unclear a diagram would be helpful.

As for the transformer not working are you sure all the original lamps worked as you may be under the transformers min' load?

 

[sbelec]

Thanks Darkwood, The only "alteration" will be to terminate the transformer correctly creating tails from the load side to the main cabling.

These light fittings consist of 2 strung wires L & N across which are the lamp holders. There is no way GU10 light fittings can be used, the ceiling is totally sealed and inaccessible so all electrics run through wall and due to height (about 8m) the lights are strung at about 2.25m. SELV was the only lghting which would have worked and there are 90 odd flats and probably all the same. It looks good.

The SELV transformers are located near the fuse box and the design of the cabling was good, 10mm cable gives a volt drop of 0.18 over 5m to the lamp cable and the lamp cable is 0.15 so the volt drop is less than 3% and the cables are man enough for the amps.

The 10mm cable goes from the transformer to a JB where it connects to the light strands.

The only "fault" is that the 10mm cable will not fit the terminals 4 strands in rest wrapped round which is how the original spark got round the problem. The replacement transformer will be fitted with tails prior to install (4mm) and this will be run to a box where it will be connected to the main cable. My short length of 4mm will not unduly affect the volt drop and all will be hunky and in complaince.

All the lamps were working and should therefore have achieved the minimum load for the transformer, I am now wondering if the 105va output of the transformer was not man enough to start the lamps from "cold" as per another thread, an tripping out on heat. I did not try removing a couple of lamps to see if that would do the trick!
Am now looking for wound transformer similar to that removed as I know that works!

steve

 

[Darkwood]

Have you checked the circuits for shorting ... it may be a reason the previous failed it may be in a lampholder or the lamp itself - if their is a short the new electronic Tx will shut-down until fault cleared.
Im not sure about lamps starting from cold been an issue, thats a new one on me and have doubts this should be a problem as thats what the Tx are designed to do.

 

[Richard Burns]

The 2m maximum length tends to be to prevent radio interference as well as covering volt drop.
The high frequency of electronic transformers means the cables act like a transmitter of radio interference, haven't seen it often happen mind you.

Have you checked the lamps to ensure that someone has not replaced some with 50W lamps, then the 105W (rather than 150W in the original post) may be exceeded, however I would still expect the lamps to come on for at least a short time.

Therefore back to Darkwood's scenario, is there an existing fault on the lighting fittings/cabling.