Cooker and hob question

m.j.w

Was fixing a problem with a cooker on Tuesday, mcb was tripped and would not reset. The cooker and hob are on the same circuit both fed from the cooker switch. Kitchen was just fitted a few weeks ago and both the oven and hob are brand new. Disconnected everything and reset the mcb, and then connected the oven, then the hob and it didnt trip. This was wired by the kitchen fitter and the connections were terrible so thought it must have been a loose connection but its after tripping again last night. Have to get the power ratings of both but the hob is pre wired with a 1.5 flex so assumed it was grand to have both on 32 amp circuit. Any ideas what the problem could be?

M cebee

doubt it's overload with hob and oven if they're up to about 10kw (total connected load )

mcb faulty?

test circuit and check joint

fault with appliances?

Diggerdunne

Find out what was on at the time.. It may have been only the hob or oven and at least then you have it narrowed down to one appliance as opposed to trying to fault find both....

m.j.w

cheers lads was talking to them there and it was just the hob they had on so will have to have a look at that

2011

m.j.w wrote: »

This was wired by the kitchen fitter and the connections were terrible so thought it must have been a loose connection but its after tripping again last night.

MCBs will trip on overload or short circuit. A loose connection at the hop or oven will not cause an MCB to trip.

Have to get the power ratings of both but the hob is pre wired with a 1.5 flex so assumed it was grand to have both on 32 amp circuit.

1.5 is quite small, but it will have a higher current carrying capacity than a normal 1.5 flex as it is a heat resistant flex.

cheers lads was talking to them there and it was just the hob they had on so will have to have a look at that

It would seem that you have narrowed it down to the hob. I would suggest that you measure the current drawn by the the hob with a grip on ammeter. This will give you a clue as to what the issue is.

m.j.w

2011 wrote: »

MCBs will trip on overload or short circuit. A loose connection at the hop or oven will not cause an MCB to trip.


1.5 is quite small, but it will have a higher current carrying capacity than a normal 1.5 flex as it is a heat resistant flex.


It would seem that you have narrowed it down to the hob. I would suggest that you measure the current drawn by the the hob with a grip on ammeter. This will give you a clue as to what the issue is.

Can a loose connection not cause an overload as when the connection heats up this heat can make its way to the bi metal strip thus tripping it on overlaod or would the heat not travel that far? Dont think its that anyway as its tripping instantly when reset. Will be heading down tomorrow after work to have a look

meercat

possibly faulty mcb(as m cebee said)
i would replace it to be sure

check also that nothing else is tapped of the cooker circuit either(turn trip off and see whats not working)

2011

m.j.w wrote: »

Can a loose connection not cause an overload as when the connection heats up this heat can make its way to the bi metal strip thus tripping it on overlaod or would the heat not travel that far?

No.

The bimetal strip in within the MCB bends as it gets hot.
The heat is generated within the MCB by excess current.
This bending causes the MCB to trip.

Heat generated from a loose connection several meters away will not cause the bimetal strip in the MCB to overheat.

Dont think its that anyway as its tripping instantly when reset. Will be heading down tomorrow after work to have a look

That sounds more like a short circuit condition.
The source of this problem could be at the connection made by the kitchen fitter.

When an MCB operates due to short circuit the bimetal strip is not used to make it trip, the magnetic properties of current are used.

Or as cebbe says it could be a faulty MCB

m.j.w

cheers lads ill let ya's know tomorrow

Bruthal

Is the MCB definitely a 32 amp one?

Edit: Just noticed in your last post it sometimes trips instantly when reset, so too small a breaker is unlikely the problem. Sounds like a short somewhere, like a nick in a cable shorting to earthed box, dodgy juinction box etc, or faulty MCB. Kitchen fitter electrical work is not often very good.

m.j.w

Went down tonight, mcb tripping instantly when reset and flash visable under one of the dials on the hob. Checked the hob flex which was grand and remade the connections in the hob terminal box and the mcb would then reset. Left the hob on for 15 mins with all the hobs on and it was grand. If it trips again does that mean there is a problem with the hob?

Yea 32amp Type c mcb so all grand there

.G.

It should be a Type B MCB. Type C will allow more current to flow before it trips on overload but that wouldn't be causing your problem since it was tripping instantly and not on overload. Possible it was a poor connection with strands of copper crossing over into N or E connection points.

Since you've remade the connections and its staying on,If it does trip again it would presumably be a faulty Hob

chiefness

1.5 is quite small, but it will have a higher current carrying capacity than a normal 1.5 flex as it is a heat resistant flex.

Not true.

1.5 mm also seems extremely small. Unless this is a tail from the hob then get rid of it immediately. Generally the oven would have a lower wattage. In any case you should use 6mm at min where the unit is not prewired.

2011

chiefness wrote: »

1.5 mm also seems extremely small.

Yes, extremely small would seem more accurate than quite small for a circuit protected by a 32A MCB. I agree with you, but the inspector from RECI that I dealt with told me that he had no issue with this at all.

Unless this is a tail from the hob then get rid of it immediately.

It is the tail from the hob according to the OP.

Generally the oven would have a lower wattage. In any case you should use 6mm at min where the unit is not prewired.

As far as I know ET101 states that a 4 sq. mm heat resistant flex is acceptable for the final part of the circuit.

Cabling with heat resistant insulation has a higher current carrying capacity that standard PVC insulation.

chiefness

Cabling with heat resistant insulation has a higher current carrying capacity that standard PVC insulation.

Would not think so. It may have a different derating factor with temp. rise.

Your point on the 4 mm heat resisting between the fixed outlet and appliance sounds sensible.

M cebee

4mm flex is ok from isolator afaik

also afaik the smaller 1.5 flex on the 'light' hob should be ok connected to the 4sq flex from the isolator

if manuf. instructions don't specify smaller mcb/fuse

2011

chiefness wrote: »

Would not think so. It may have a different derating factor with temp. rise.

I can assure you this is the case.

By using different types of insulation the current carrying capacity of a cable can be increased.

For example:

Refer to ET101

Look at table A52-F1 for a 6 sq. mm PVC cable.
Installation method C
CCC = 46A

Look at table A52-F5 for a 6 sq. mm MICC cable.
Installation method C
CCC = 57A

Your point on the 4 mm heat resisting between the fixed outlet and appliance sounds sensible.

Thanks, but I can't claim credit for that (it is in the regs).

2011

M cebee wrote: »

if manuf. instructions don't specify smaller mcb/fuse

Yes, the key issue is that the MCB should operate before the maximum current carrying capacity of the cable is exceeded.

Bruthal

chiefness wrote: »

Would not think so. It may have a different derating factor with temp. rise.

It would imo. A cable with heat resistant flex can carry a higher current before the insulation would be affected, compared to normal pvc. So regardless of whether the rules allow for it, the cable would have a higher capacity.

Multi strand flexible cables also have a higher capacity than a solid one would.

chiefness

2011 wrote: »

chiefness wrote: »

Would not think so. It may have a different derating factor with temp. rise.

I can assure you this is the case.

By using different types of insulation the current carrying capacity of a cable can be increased.

For example:

Refer to ET101

Look at table A52-F1 for a 6 sq. mm PVC cable.
Installation method C
CCC = 46A

Look at table A52-F5 for a 6 sq. mm MICC cable.
Installation method C
CCC = 57A

Your point on the 4 mm heat resisting between the fixed outlet and appliance sounds sensible.

Thanks, but I can't claim credit for that (it is in the regs).

Excuse my ignorance but I've yet to see a cooker or hob wired in micc?

chiefness

robbie7730 wrote: »

chiefness wrote: »

Would not think so. It may have a different derating factor with temp. rise.

It would imo. A cable with heat resistant flex can carry a higher current before the insulation would be affected, compared to normal pvc. So regardless of whether the rules allow for it, the cable would have a higher capacity.

Multi strand flexible cables also have a higher capacity than a solid one would.

Solid cable/stranded will always have a higher ccc then flexiblle so you are incorrect there.

Bruthal

2011 wrote: »

Yes, the key issue is that the MCB should operate before the maximum current carrying capacity of the cable is exceeded.

It depends on what you refer to as the current carrying capacity. The max current the cable can conduct, or the max it can continuously carry.

In the case of flexes to appliances, it is possibly assumed they wont likely be subjected to a continuous overload, just a short circuit, in fault conditions.

Bruthal

chiefness wrote: »

Solid cable/stranded will always have a higher ccc then flexiblle so you are incorrect there.

That would be news to me I must admit. I would be confident that multi stranded has a higher capacity for AC current, although probably negligible at 50hz. But solid certainly wont be higher.

chiefness

robbie7730 wrote: »

chiefness wrote: »

Solid cable/stranded will always have a higher ccc then flexiblle so you are incorrect there.

That would be news to me I must admit. I would be confident that multi stranded has a higher capacity for AC current, although probably negligible at 50hz. But solid certainly wont be higher.

I suppose you are correct technically but it's a lot easier to lug two solids then one flex.

2011

chiefness wrote: »

Excuse my ignorance but I've yet to see a cooker or hob wired in micc?

I have not seen it either:D

I was using this example to illustrate the fact that by using different types of insulation the current carrying capacity of a cable can be altered.

2011

robbie7730 wrote: »

That would be news to me I must admit. I would be confident that multi stranded has a higher capacity for AC current, although probably negligible at 50hz. But solid certainly wont be higher.

I suspect you are correct as stranded conductors have more surface area than solids, thus increasing the cooling effect.

2011

chiefness wrote: »

I suppose you are correct technically but it's a lot easier to lug two solids then one flex.

Lugs are not permitted on solid core copper cables.

chiefness

2011 wrote: »

chiefness wrote: »

I suppose you are correct technically but it's a lot easier to lug two solids then one flex.

Lugs are not permitted on solid core copper cables.

Have you ever wrapped a 120mm solid around a terminal?

2011

robbie7730 wrote: »

It depends on what you refer to as the current carrying capacity.

I would consider the current carrying capacity of a cable to be the maximum current that it can sustain indefinitely.

According to 433.1 of ET101, the design current of the cable shall be < or = the rated current of the protective device which shall be < or = the continuous current carrying capacity of the cable (according to 523).

In other words, the protective device operates before the cable fails.

2011

chiefness wrote: »

Have you ever wrapped a 120mm solid around a terminal?

No, but I have terminated them by a suitable means that complies with the regulations.

chiefness

2011 wrote: »

chiefness wrote: »

Have you ever wrapped a 120mm solid around a terminal?

No, but I have terminated them by a suitable means that complies with the regulations.

That sounds like a Vicky Pollard Yes/But No.