Low voltage light tripping when switched off....!!

[spark-ticus]

Hoping the wealth of knowledge can shed some light on this..............Have fitted 6 sets of "cable track lights" each set consists of 2x 5m cables spanned parallel across the ceiling with 5x 35w fittings screwed to cable piercing the insulation to obtain L-N. Each set has a transformer with the following spec; PRI 230v - 240v ~ 50Hz SEC 12VAC ~ 17.5A 210VA
I have wired 2 sets to 1 swich in each case, but after i have turned the lights on and they light up beautifully i then turn them off and they trip the breaker (10A type B mcb). Any ideas on what is happening would be greatly appreciated......

 

[westom]

What type of mini breaker? RCB type?

 

[Pete999]

Inrush to the transformer try a c type

 

[DPG]

Inrush to the transformer try a c type

Switching OFF though, not on. Inductive 'back emf' from the transformer's magnetic field collapsing? Daz

 

[Silly Sausage]

Switching OFF though, not on. Inductive 'back emf' from the transformer's magnetic field collapsing? Daz

Hmmmm, like your thinking but looking at the graph, 50 Amps within 10 seconds....

 

[Lucien Nunes]

I wonder whether the switches are failing to break the circuit cleanly and the contact bounce is giving the MCB multiple hits of inrush while the rocker is moving to the off position. I.e. it's not the breaking of the circuit that does it. An AC micro-break switch continues to carry current while switching off until the current-zero at which the arc extinguishes and the circuit opens. It's a bit far-fetched to think that it would then re-make the circuit but if the switch action is really rough it's the best explanation I can think of without more info.

Back-emf is the release of the stored energy in the transformer core that occurs if you catch it at the wrong point on the mains cycle. However although the voltage into an open-circuit can be high, the current should not normally exceed the peak current when energised, so I am sceptical how that could trip the MCB (although it can cause chaos with voltage-sensitive things). OTOH the inrush that occurs when you switch a transformer on at the wrong point in the cycle is caused by magnetic saturation and the current can be very high because the inductance momentarily drops. Hence my looking for some explanation as to how the circuit might be getting re-made while switching off.

Barrage of questions, sorry:

Does the MCB ever trip at switch-on?
Does it always trip at switch-off?
Does the transformers sometimes make an audible thump when switching on or off?
Do all three 'pairs' behave the same?
What type of switches are in use?
How near to 10A is the MCB running?
Is there anything in circuit that might contain a VDR surge suppressor (that the back EMF is kicking into conduction)?

And one final one, if you didn't wire the switches yourself, have you checked that L2 hasn't been used to joint the neutral?

 

[spark-ticus]

hi guys sorry haven't been back sooner, Thankyou all for your input, Additional info as follows

mcb has tripped at switch on if it is switched on too soon after it has been turned off ie 2-5 secs
difficult to hear any audible thump on transformers due to position of switch in relation to position of transformers (can go back to listen for this tho if its crucial info)
All 3 pairs behave the same in that they trip the breaker but 1 pair is every single time where as the other 2 are once every three or four times
mcb running at approx 50% capacity
surge suppressors are outside my band of knowledge, can i have an example of one..?

hope this helps you to help me

 

[spark-ticus]

".............also light switches are standard 2way switches 10A white plastic

 

[Darkwood]

You have multiple inductive loads on one switch and its down to poor design, as lucien has already given a more indepth explanation i will refrain from doing so but totally agree the back EMF is too large and it will be arcing across the opening switch contacts creating spikes and other than the tripping issues the switch contacts will be buggered in no time.

Run extra switch wires to reduce the combined inductive spike and also add an additional MCB - as its tripping every time it would be a tell tale sign you need to spread the load due to their inrush nature. If they are adamant on 1 switch then set-up a contactor suitable for inductive lighting loads and run your circuits through it having the switch just pulling the coil in.

PS LV lighting- people still use that?.... I stop using them several yrs ago they used to have lamp life benefits against mains versions but nowadays times have changed we have better mains alternatives that are more flexible and versatile.