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#53362 06/22/05 03:04 PM
Joined: Jan 2004
Posts: 54
Last week we installed some low voltage linear lighting in some display cases in a library. We removed some old fluorescent strips which were existing and used the original circuit and 2 switches that controlled the lights.

The new lights, which were supplied by the building manager, is a 24 volt system. It now consists of 2 600 watt transformers with built in circuit breakers feeding approx. 10' of lighting per transformer.

Everything fired up fine, then I get a call the lights are out. It turns out the breaker has tripped. I check the load on the circuit and each transformer is drawing about 3.5 amps, total circuit 7 amps. So I decide to change the older SqD QO bolt on breaker.

After changing the breaker I'm discovering that the circuit still trips especially when I rapidly turn the switches on and off and not 1 particular switch. Either switch being turned on and off rapidly will cause this to trip.

I need to return tomorrow morning before the library opens to dig a little deeper. I'm wondering if a problem on the neutral might be the problem however there does not seem to be a logical explanation. The wiring is BX in very good shape (the building is about 30 years old).

Any suggestions would be appreciated.


#53363 06/22/05 04:20 PM
Joined: Feb 2002
Posts: 182
Bob Offline
You need to provide more info.
1 Are the transformers 120/24 volts? Are they on the same circuit?
2 "It now consists of 2 600 watt transformers with built in circuit breakers feeding approx. 10' of lighting per transformer" Is this the breaker that tripped or the line circuit breaker?
3 What size is the circuit breaker?
4 Is the amperage measured on the 120 volt side?

These types of transformer can draw large inrush currents. Sometimes 40 x FLA. If you have a good voltmeter you can measure the max current and see.

#53364 06/22/05 04:35 PM
Joined: Jan 2004
Posts: 54
Bob, thanks for responding.

The transformers are 120v primary, 24v secondary.

Yes, they are on the same 120 volt circuit.

The 20 amp SqD bolt single pole breaker is tripping.

The "breakers" on the individual transformers have not tripped.

I measured the amperage on the primary side at the circuit breaker using a clamp on amp probe.

#53365 06/22/05 04:38 PM
Joined: Jan 2001
Posts: 1,044
Tom Offline
I think inrush current may be the problem. Square D makes some breakers called "high inrush." I've had to use these on some multiple ballast signs that tripped the breaker(s) for no detectable reason.

The stock # is QOB120HM

Good luck


Few things are harder to put up with than the annoyance of a good example.
#53366 06/22/05 07:53 PM
Joined: May 2003
Posts: 2,876
e57 Offline
I'm with Bob and Tom here.... You probhably notice a "ping", or "pong" when you flip the switch? How many watts are you running on each transformer?

Are you using a decent "lighting" Transformer, or one of those buck and boost T's with a few modifcations of adding CB's from a "Manufacturer". Some of the cheaper ones will just modify a buck and boost. That will give them a very high in-rush. (As they were designed for power, not lighting...) You can try the breaker suggested, or if possible use a dimmer to ramp up the switching current, or switch seperate.

Mark Heller
"Well - I oughta....." -Jackie Gleason
#53367 06/22/05 08:16 PM
Joined: Jan 2004
Posts: 54
I think you guys are right on the money with this. I'm going to try to locate that breaker tomorrow and give it a shot.

The transformer is made by the same manufacturer as the linear lighting. It is a similar system to Juno Trac 12 but the name escapes me right now.

I'll post back and let you all know how it works out.

Thanks a lot for the info!

#53368 06/22/05 08:36 PM
Joined: May 2003
Posts: 2,876
e57 Offline
Juno LV tansformers are buck and boost's with black paint, and a CB added...

Mark Heller
"Well - I oughta....." -Jackie Gleason
#53369 06/22/05 08:52 PM
Joined: Jun 2005
Posts: 124
This is good stuff, you guys amaze me with the amount of suggestions that so much collective experience makes possible. Really glad to have found this site!

#53370 06/22/05 09:44 PM
Joined: Dec 2001
Posts: 444
Wouldn't installing one of those Maestro touch dimmers where the lights turn on and off gradually (theatre dimming) make a difference?


Oops...just reread the thread, I see E57 already suggested the dimmer thing. Sorry.

[This message has been edited by Sandro (edited 06-22-2005).]

#53371 06/22/05 10:07 PM
Joined: Jan 2004
Posts: 615
I have had Meastro's fail because of a little plastic piece behind the plate. I would leary of using it in a commercial environment unless it was to be used by a soft, gentle librarian.

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