Breaker Slow To Trip

[just pumps]

Morning all,

Friend has brought a boat and found that when the internal lights are switched on the breaker trips.
It is a 12v circuit, the lights themselves all seem ok and a boatyard suggested that a screw has cut through a cable but he hasnt added anything fitted with screws.

What I dont understand is why there is a slight delay before the breaker does trip. One last thing, there seems to be two seperate cable runs powering the lights.

Any ideas guys (or gals)

 

[carmanmemoranda]

How many lights are on the circuit?

Is it 1 specific light that makes it trip ? Or when all, or so many are on ?

 

[just pumps]

Think there are around six lights and when any light is turned on. Haven`t actually been there.

 

[Harry Bloomfield]

It is a 12v circuit, the lights themselves all seem ok and a boatyard suggested that a screw has cut through a cable but he hasnt added anything fitted with screws.

Are you saying a 12v 'breaker' trips, or a 240v 'breaker' trips? What is the 'breaker', exactly? Is it a MCB, RCD, or RCBO. A photo might help.

Screws can gradually work their way into cables, causing faults, long after the screw is put in.

 

[just pumps]

I doubt if she will know if it a 12v or 240v breaker tbh. Please consider either in the replies for now.

 

[Harry Bloomfield]

I doubt if she will know if it a 12v or 240v breaker tbh. Please consider either in the replies for now.

Sorry, that makes for too many options... You do really need to find out exactly what is tripping..

 

[just pumps]

Work on it being 12v as the lighting systems on boats are all 12v.

The question was, Why does the breaker seem to hesitate before tripping.

 

[JohnD]

Any ideas guys (or gals)

Save up for a multimeter to see what the load is.

If it is due to lighting load, changing to LED lamps will greatly reduce the load.

 

[Harry Bloomfield]

Work on it being 12v as the lighting systems on boats are all 12v.

The question was, Why does the breaker seem to hesitate before tripping.

A slow to trip 12v 'breaker', could point to the 'breaker' simply being close to it's maximum current rating. Alternatively, it might simply be a failing 'breaker'. The way to find out which, would be to measure the current demanded by the circuit.

Does the 'breaker' still trip, if one or more of the lamps are unplugged?

 

[JohnD]

The question was, Why does the breaker seem to hesitate before tripping.

Ordinary MCBs have a thermal and a magnetic trip.

The magnetic trip is fast, on a severe overload.

The thermal is slow, on a small overload, as it heats up over time, and will allow brief, small overloads.

The smaller the overload the slower the trip

(Another method of using delay is a magnetic slug moving in a cylinder of viscous fluid, against a spring, the fluid makes it move slowly)

The delay can also be caused by the start-up characteristics of some equipment coming online.

 

[ericmark]

12 volt often look something like this

and are thermal only, where as 230 volt tend to look like this and are both thermal and magnetic, but for the magnetic part to work, the current is typically 5 times the rating, so with a boat a little pointless as unlikely that amount of current can flow for an inverter.

So yes with a thermal only device it will be slow, as it takes time to warm up.

Traditionally we would use an ammeter like this but today the cost of hall effect ammeters has dropped, so one like this is now actually cheaper than the first one. So looking at around £35 for a DC clamp-on ammeter, the old Durite one still around £43 for 30-0-30 (0-534-30 part number).

 

[Harry Bloomfield]

12 volt often look something like this

and are thermal only

No Eric, some are magnetic.

 

[just pumps]

Thanks for the replies. Being on a boat the wires are well hidden as you would expect and to make matters worse wires hidden by the roof lining (which is hard to remove and refit) tend to be fixed to batons with little or no free play in the wires.

The boat yard has suggested a screw through a wire but it was all working fine for the first 24hrs and she say`s that she hasn`t done anything. Would I be right in thinking that a screw causing the short would lead to the breaker activating instantly.

 

[ericmark]

No, a thermal breaker it would depend on the resistance of the wires.

 

[Harry Bloomfield]

The boat yard has suggested a screw through a wire but it was all working fine for the first 24hrs and she say`s that she hasn`t done anything. Would I be right in thinking that a screw causing the short would lead to the breaker activating instantly.

Most likely - yes, but not definitely. As I suggested, try removing the lamps, to see if still trips - it could well be too many lamps, on the circuit, causing the trip. If it trips, with no lamps, then you have a bigger problem....

You need to work out how it is wired, split the wiring into sections, test each section for shorts, then progress from there.

Without more information about the system, we can only guess, and guesses can be very wide of the mark..