Question for sparkies (3-phase problem)

[Rossluck]

I have what for me is a strange electrical problem. Please note that I don't do my own electrical work, but when I have a problem I like to identify it before I call the sparkie. This is because they are so busy in this area that you have to convince them that it's only a small job....

Tonight I was giving my welder a hiding to cut some plate (three-phase stick running at 250 amps) and I lost power. I thought it was an earth-leakage or overheating trip, but it wasn't. After carefully checking with my meter (a good one), I've found that one phase is running at around 35 volts. This happens to be the phase that the 240 is taken from in the shed, so it was also running a small AC and a radio.

The other two phases are normal at just over 233. I traced this reading back to the main power box (on a pole on our property), but couldn't access a neutral to test it there (I reached the end of what I'll do).

I know I'll have to call a sparkie, but have you any idea what may have happened? We also have three phase running to the house and a 3-phase AC, and they're OK, so I assume this problem is isolated to the shed's circuit breaker. Do circuit breaker's do that? Just fail on one pole?

[thatirwinfella]

cb's shouldn't fail on one pole, 3 phase breakers are generally interlocked so they trip simultaneously.

there might be an open circuit somewhere causing a high resistance path and increasing the voltage drop. it could be anything from a loose screw to a rat tooth.

if a phase has dropped all the way back to the pole, it very well could be a powercor/distribution problem. try calling them first to see if they have reports of a fault in the area.

[Rossluck]

there might be an open circuit somewhere causing a high resistance path and increasing the voltage drop. it could be anything from a loose screw to a rat tooth.

.

Thanks thatirwinfella. Could you elaborate on this a little? An open circuit where? In my shed? The welder?

[Feralbilly]

I had a similar problem in a house I was renting in Darwin. Loss of power to some GPOs and Ok on others. Of course it was "after hours" so called a sparkie out and he checked at the meter box and said "you have dropped a phase, call Power and Water."
When I called Power and Water and told them I had dropped a phase to the meter box, their response was "You can't have. You don't have three phase at that address."
I told them that a sparkie had checked it out and told me it was 3 phase.
Anyway a crew came out and said "Yes, it is 3 phase and one of the phase fuses at the top of the power pole in the street had blown.
Up in the cherry picker, replace the fuse and, voila, all back to normal.

Bill

[Jim Carroll]

You claim the AC in the house is ok.
Then your problem can be in the isolator in the shed or the one in your main box feeding the shed.
I have had the main one break down but not shut down with a similar result, 2 phases ok and one only feeding a small voltage. The breaker did not trip because there was no short circuit.
Replaced with a new on and no more problems.
Get the electrician to check your load on each circiut, you may be overloading one and causing the fault.

[wattlewemake]

Well you could have a few different problems.
1. Your circuit breaker could of burnt out one pole causing a loss of a phase. It is possible.

2. You have a bad cable between your house board and your shed. Highly unlikely.

3. Your supply fuses down the street might of popped. Very possible.

4. You could have a problem in your main switchboard. Possible.

Usually they dont run all 3 phases into your house. Mostly split it across 2 if there is enough load. Eg air cons and the like.

When checking 3 phase with a meter, check it to neutral. Should have 240v. Check it between phases. Should have 415v. If you have 240v or somethin like that you have a missing phase or similar. Check phases to earth of your switchboard.(metal frame of switchboard. Obviously dont do this if you have a plastic board) Should have 240v. If in any doubt get a qualified person to test it for you.

Hope this helps.

Shane.

[Ashore]

Test as shane says but do it before and after the circuit breaker in the shed , and yes you can burn the contacts on one leg of a three phase breaker without loosing the other two legs

Rgds

[Rossluck]

Well, for some strange reason the damn thing is running again. It sounds for all the world like a supply phase dropped out, but when I was doing my checks I noted that all three meters were running. Moreover, the houses's power was fine, and the power is spread across the three phases.

This afternoon I had better light, so I checked the power at the main fuse box and had full power above and below the shed's circuits breaker. This had me worried, because I assumed that the break must be underground between the power pole and the shed. But when I checked the circuit box in the shed, I had full power on all three phases there as well.

I was interested in thatirwinfella's theory of an open circuit, thinking that I may have welded a connection in a switch or something, but then I realised that I had disconnected the suspect wire from the circuit breaker in the shed, so the problem had to be between the shed and the mains.

It's a worry when they just start working again without an explanation. It gives me the jitters. Time to call a sparkie!

[thatirwinfella]

by open circuit i meant a connection may have come loose, possibly through vibration/movement or not just not being physically continuous. if a quality contact doesn't exist it causes voltage drop through higher resistance then the rest of the current path.

but this can also be caused by anything from a shovel cutting an underground cable in half or a rats tooth doing something similar.

if it's working again than that suggests two possibilities.

1- the power to one phase was out at the supply end and they've fixed it.

or

2- an intermittent open circuit is occurring when the circuit is in use. it could be that that phase is overloaded and heating up, causing it to open circuit, and when left alone long enough it cools and closes circuit. this is not good.

the next time it happens, ask your neighbours if they've lost power. typically they'll be on a different phase as by the law of averages the phases are then balanced. one of your neighbours should be in the dead phase.

a bad breaker sounds possible also, if you can try using the welder on a different circuit.

a sparky does sound like a good idea

[Rossluck]

by open circuit i meant a connection may have come loose, possibly through vibration/movement or not just not being physically continuous. if a quality contact doesn't exist it causes voltage drop through higher resistance then the rest of the current path.

but this can also be caused by anything from a shovel cutting an underground cable in half or a rats tooth doing something similar.

if it's working again than that suggests two possibilities.

1- the power to one phase was out at the supply end and they've fixed it.

or

2- an intermittent open circuit is occurring when the circuit is in use. it could be that that phase is overloaded and heating up, causing it to open circuit, and when left alone long enough it cools and closes circuit. this is not good.

the next time it happens, ask your neighbours if they've lost power. typically they'll be on a different phase as by the law of averages the phases are then balanced. one of your neighbours should be in the dead phase.

a bad breaker sounds possible also, if you can try using the welder on a different circuit.

a sparky does sound like a good idea

I called the sparkie who wired the shed. I caught him at a bad time, he's retired from electrical work and is in Bundaberg, but he was very helpful.

He suspects a bad breaker at the main fuse box. He said he's experienced it before. His theory is that when I was using the welder I overheated the main circuit breaker and bent some internal connection out of position. When it cooled the connection returned to something of it's original position and restored power. Sounds good to me.

He's coming on Saturday to check and replace it.

Thanks everyone for the usual great assistance.

[spartan]

Given your location, the time of year, and the weather I'd think that if its not in your box it is a transformer problem in your street.
We've been replacing tranformers non stop for two years due to low volatage as more and more people increase their electricity usage.

The gold coast has one of the worst supplies in the state.....