ring main overloading

[andy]

was at a friends house earlier where i took the cover off the CU to be greeted by 2 red, partly burnt cables (aprox 15mm from where it goes into the MCB). itz a pretty big house and only has 1 ring for the whole house (apart from the kitchen). i checked that the live wasnt broke. my question is: the burnt cable would obviously be b/c of overloading, but why didnt the breaker trip? (he said it has never triped)

ne ways, am gonna be splittin the ring into 2, one for up and one for down

 

[plugwash]

burnt wires at a single location are unlikely to be caused by overloading

more likely they are caused by a bad contact (which has high resistance and therfore gets hot)

 

[andy]

they did have good contact with the MCB tho. i didnt have time to check other sockets tho to see if they were burnt. however, there is a room in the house what does have high power usage every so often which i think could be causing the overloading.

another thing i noticed was the meter is rated at 40A. should this be replaced?

 

[Damocles]

The rating of electricity meters is a mystery known unto the supply company. What is the accompanying fuse rating, and size of meter tails/earthing?

As the man said, heating at just one place is likely to be because of a bad connection. Conceivably the wires could be screwed in tight but corroded or the insulation was trapped? Also, bad contacts inside a switch can cause heating.. which must include inside a mcb, though I have no idea how common such a thing might be.

How high is this high power usage? Dr. Frankensteins home lightning kit?

Circuit (presumably) 32A wired in 2.5mm. Would the load be greater than this? Is the load all concentrated at one end of the ring, which might mean an excessive proportion of the current is going down one leg of the circuit.

 

[andy]

The rating of electricity meters is a mystery known unto the supply company. What is the accompanying fuse rating, and size of meter tails/earthing?

the fuse is 60A. the tails between the fuse and meter appear to be 25mm altho the calb ebetween the meter and the CU is a T+E, possibly 16mm. for the earth, there is none coming from the service fuse, however, there is an isolator between the meter and CU where an earth (with green insulation) comes out (form the in side) and goes to the CU

As the man said, heating at just one place is likely to be because of a bad connection. Conceivably the wires could be screwed in tight but corroded or the insulation was trapped? Also, bad contacts inside a switch can cause heating.. which must include inside a mcb, though I have no idea how common such a thing might be.

the copper ends are clean and no insulation is caught

How high is this high power usage? Dr. Frankensteins home lightning kit?

not sure, but it has loads of disco stuff. forgot to take my clamp meter but al be back there on tuesday. total power usage in that room i think will be between 15 and 20 amps.

Circuit (presumably) 32A wired in 2.5mm. Would the load be greater than this? Is the load all concentrated at one end of the ring, which might mean an excessive proportion of the current is going down one leg of the circuit.

the circuit is on a 32A breaker with 2.5mm cable. unsure of the route taken for the ring, but the room in question am gessin would be at the centre of the ring. the total load could go over 32A, esp when all the dicso equip is used, adding to the total load on the ring from the other rooms

 

[Damocles]

Doesn't sound too unusual, though a slight mystery where the extra earth is coming from. Wouldn't like to think that what you imagine is the source of the earth is actually an earth being taken TO the isolator from the CU. Maybe more wires behind from the wall?

A ring should be able to take what you throw at it. It is designed to work safely under full load, and trip when this is exceeded by more than the built-in safety margin. Having said that, you might check it does not have an electric cooker, shower and immersion heater all connected onto it.

If cables are burnt then there is some kind of problem. Possible it is an old fault already repaired? You could try running everything for an hour or whatever is normal, switch off power and see what is hot.

 

[andy]

Doesn't sound too unusual, though a slight mystery where the extra earth is coming from. Wouldn't like to think that what you imagine is the source of the earth is actually an earth being taken TO the isolator from the CU. Maybe more wires behind from the wall?

i think the earth might be taken from the neutral in the isolater. i took the cover off and had a quick look, and there was a terminal between the phase and neutral whcih had the earth connected to it. didnt want to poke round in it much tho since it was live

Having said that, you might check it does not have an electric cooker, shower and immersion heater all connected onto it.

the cooker and shower have their own circuits and i dont think there is an immersion heater

If cables are burnt then there is some kind of problem. Possible it is an old fault already repaired? You could try running everything for an hour or whatever is normal, switch off power and see what is hot.

the CU is fairly new (approx 18 month). i doubt there would be burnt cables out in to it. i will be back round there on tuesday to split the ring. will also find out what the power useage is.


another question is the meter is rated at 40A. what would happen to it if more than 40A went through it? if the shower and cooker were both on added to the power used by the ring, the total would go well over 40A

 

[Damocles]

I imagine either the meter would blow up or work fine. As no one has had a mad scramble to replace it, I gues they work pretty well ok despite the rating. It might be an accuracy rating rather than power handling?

Though round out way they had a burst of replacing meters every couple of years.

 

[andy]

if the meter was overloaded (i.e 50A) could it cause it to over-read (as in registed 52A goin thru)?