Troubleshooting secondary 12V circuits

[sparrott4]

Every now and then I find intermittant tripping of a secondary magnetic circuit breaker in a low voltage outdoor lighting system.

The cause has sometimes been a buried low voltage wire that has been nicked by a shovel or trimmer. It could be that the presence or absence of moisture in the soil determines whether or not the breaker trips.

My questions are:

What is the mechanism involved when a nicked wire trips a breaker? Can it only happen when both legs are nicked - creating a short (perhaps across moist soil)?

If only one leg is nicked (bare wire in contact with soil) can this cause an increase in amperage that leads to tripping the breaker?

Can a nicked wire be detected by measuring voltage at the lamp or amperage back at the transformer?

Thanks

 

[hbiss]

Given that the OCP on a 12v circuit is many amps, the likelihood of that being conducted at 12v via even wet soil surrounding a nick in the insulation is nil. I would say that you have direct conductor to conductor shorts.

-Hal

 

[TLBryant]

Perhaps the Lighting transformer has GFCI built in to the secondary OCP since it is intended to operate outdoors and in contact with earth.

Also, it is possible that splices on both legs are coming in contact with water or wet soil at the same time causing a short across the conductors.

These systems often have tap splice assemblies that puncture and clamp to the zip cable that could cause both conductors to be exposed to moisture at the same place.

 

[sparrott4]

No GFCI on the secondary and splices are not suspect.

What happens if a single bare wire contacts the soil - voltage loss? Increased resistance?

 

[LarryFine]

What is the mechanism involved when a nicked wire trips a breaker? Can it only happen when both legs are nicked - creating a short (perhaps across moist soil)?

Extremely unlikely without direct metal-to-metal contact.

If only one leg is nicked (bare wire in contact with soil) can this cause an increase in amperage that leads to tripping the breaker?

Even more unlikely.

Can a nicked wire be detected by measuring voltage at the lamp or amperage back at the transformer?

No, because the low-voltage output is, or should be ungrounded.


If the breaker trips instantly, there's a short; if it takes a few minutes, there's an overload. What's the total wattage of the bulbs compared to the transformer's capacity?

Remove all of the bulbs and take a resistance reading, and have someone watch the meter while you mess with the connections. Then, replace the bulbs, one at a time.

The breaker may not be tripping because the current is really that high. It may be bad and is tripping too easily. Do you have a high-current ammeter?