24V trouble shooting

[Frig Pig]

Small multi-zone running off on transformer. O.K. the in series 3A breaker is cutting out. because the common is grounded on the circuit how does one go about finding a fault and what is the best way to test the circuit for high res. short of removing each load from the circuit and retesting? BTW i finally found a coil in the heating section that had abnormally high res. there has to be a faster way to do this. i took me close to 4 hours to check every load.

[rbartlett]

you can always brush up on your Taguchi method

http://www.amsup.com/taguchi_methods/

cheers

richard

[Bones]

well, first check for hot joints cause with low voltage control circuits hot joints in wiring will increase resistance (sounds like u done this already). if that checks ok, test resistance of each load as you mention.

[Mark]

hi frig pig
After isolating electrically i would do a visual check on components,assessing the contents of the circuit at fault,
The cause of the circuit breaker tripping out could be due to overcurrent conditions or earth leakage.Faulty breaker(etc)
Without segregating and testing components until the fault has clearly gone by means of insulation resistance testers.(do not test LV, pcbs, electronic equiptment with a insulation tester or meggar as this will damage it) .
There is no short cut to fault finding electrical circuits safely.
Certain wiring regulations vary but i would rather see infinite resistance when testing components to ground(earth leakage).
Remembering to disconnect neutral conductors (where applicable)
as would cause a parallel path and you would end up chasing the fault about.
hope this does not insult anyones intelligence
regards mark

[chemi-cool]

Hi Frig Pig,

A lot of patient is needed when you look for electrical problems. Every tech. develope his oun procedure, some go straight to control circuit and some start with power.

Whats important is to do it thorough and not to neglect any component.
With time, one's learning to go many times directly to the foulty component.

It will happend to you many times that you will be called to check a unit without any records or diagrams and I think its the best school ever.

Good luck in the future.

chemi

[750 Valve]

Just my 2 cents worth... find out if you're dealing with earth leakage or circuit short circuit/fault. Easy way is to megger it, but like mark says don't megger circuit boards.

[RogGoetsch]

Originally posted by Frig Pig
Small multi-zone running off on transformer. O.K. the in series 3A breaker is cutting out. because the common is grounded on the circuit how does one go about finding a fault and what is the best way to test the circuit for high res. short of removing each load from the circuit and retesting? BTW i finally found a coil in the heating section that had abnormally high res. there has to be a faster way to do this. i took me close to 4 hours to check every load.

Scratching my head over this. Assume you mean the transformer is drawing too many amps. But high resistance in the circuit? I would check the current draw on a solenoid device but not resistance. What am I not understanding?

Is it instantaneous? If not, I had one where the installing contractor had tied an additional pneumatic solenoid valve to a package unit and the transformer was overloaded. Would fail shortly after all loads energized. Found out by adding up the rated draw for each device.

[Frig Pig]

It was popping after a few seconds. Small noise was made too. I suspected a load in the elec. panel but the humm turned out to be the transformer over amping. I’m not curious how to figure out this problem.... I did... just how to make finding a problem like this systematic and less time consuming.

[RogGoetsch]

But what did you find? A coil with high res.? What does that mean? A stuck solenoid of some kind will cause its coil to over-amp. Is that what happened?

In my early days I was aghast that an old-timer bypassed the fuse in a similar situation. He pointed out to me the difference in the price of a control transformer and the cost per hour to the customer for his labor.

If the trip is not instantaneous, you're probably not looking for a short or even a ground-fault. Transformers are rated in volt-amps (VA) so a quick amp-check will tell you if you can leave it in the circuit long enough to amp the loads.

Even if you don't know the rated draw for a load, you should have more than one to compare.

If it's a ground fault you're looking for, you can disconnect the transformer ground while you test.

[Frig Pig]

I found a relay coil in the heating section that had a strangely high resistance. I took it apart and found the coil burnt up in one small area. So I replaced it and presto! Another successful service call. I just took forever too find.