Oven Hob Flex protection

[MJS9691]

We have a new ceramic hob (6kW) factory prewired in 2.5mm flex and oven (2.6kW) prewired in 1.5mm. After diversity calcs the existing supply 32A protected 6mm (9m run) suggest a more than adequate supply for both appliances however:-

Some electricians are suggesting further protection of the flex beyond the outlet plate to protect the individual cabling on the appliances with FCUs and some say its OK to wire directly into a dual outlet plate with no further protection. Some Electricians are even suggesting removing the factory fitted cables and replacing with 6mm right up to the appliance.

Is there any definitive rules on this, or is this purely down to the electricians choice i.e. is it necessary or not to protect the 1.5mm flex cabling in the event of a fault on the appliance and it somehow drawing more current than the cable can take. There is no information on appliance fusing or protection in the installation manuals of either appliance.

Views on this matter would be appreciated as it still appears to be a grey area reading through the posts with installations not standardised...

Many thanks

 

[Engineer54]

In Europe, only the UK has the facility of cord fuse protection via either a plug top or FCU. No-one ever seems to wonder how these same appliances (The exact same appliances will be available in the European markets, ...and beyond) are to be connected in those countries!!! lol!!

Seeing as both the cooking appliance you are referring too can be considered as fixed loads, (eg, ....cannot overload) the cords to these appliances cannot be stressed beyond their CCC due to overloading. The 32A MCB will quite happily look after any short circuit fault on the circuit!!

 

[snowhead]

As above, Not needed.

How far do you go.
Inside the cooker will be cables possibly less than 1mm, would the "Electricians" want to change them as well?

If the flex and appliance needs Fused protection it will be in the Manufacturers instructions and you've already looked in there.

 

[telectrix]

same as what him ^^^^ and him ^^^^^^ said.

 

[Engineer54]

We have a new ceramic hob (6kW) factory prewired in 2.5mm flex and oven (2.6kW) prewired in 1.5mm. After diversity calcs the existing supply 32A protected 6mm (9m run) suggest a more than adequate supply for both appliances however:-

Some electricians are suggesting further protection of the flex beyond the outlet plate to protect the individual cabling on the appliances with FCUs and some say its OK to wire directly into a dual outlet plate with no further protection. Some Electricians are even suggesting removing the factory fitted cables and replacing with 6mm right up to the appliance.

Is there any definitive rules on this, or is this purely down to the electricians choice i.e. is it necessary or not to protect the 1.5mm flex cabling in the event of a fault on the appliance and it somehow drawing more current than the cable can take. There is no information on appliance fusing or protection in the installation manuals of either appliance.

Views on this matter would be appreciated as it still appears to be a grey area reading through the posts with installations not standardised...

Many thanks

It's only a grey area for those that don't understand the fundamentals, and/or can only install by numbers or by following pretty pictures in trade guides etc... lol!!

 

[MJS9691]

It's only a grey area for those that don't understand the fundamentals, and/or can only install by numbers or by following pretty pictures in trade guides etc... lol!!

Thanks for the replies guys, much appreciated. That's what I suspected; however regarding grey areas, I also posted the very same question on another forum and this is the answer I got below, so as you can see still lots of confusion, even amongst professionals; so to protect or not protect when using a dual outlet plate; that is the question?

Answer: If its a silicone rubber HORN-F type flex then even though its only 2.5mm it can take a higher current than a standard 2.5mm flex. This said these appliances are designed for the mainland European market and not specifically for the UK. In Europe its one circuit dedicated per appliance and not two linked together. For the 2.6KW a 13amp fused spur could be used.
Submitted by: Electrical Safety Services

 

[Engineer54]

Thanks for the replies guys, much appreciated. That's what I suspected; however regarding grey areas, I also posted the very same question on another forum and this is the answer I got below, so as you can see still lots of confusion, even amongst professionals; so to protect or not protect when using a dual outlet plate; that is the question?

Answer: If its a silicone rubber HORN-F type flex then even though its only 2.5mm it can take a higher current than a standard 2.5mm flex. This said these appliances are designed for the mainland European market and not specifically for the UK. In Europe its one circuit dedicated per appliance and not two linked together. For the 2.6KW a 13amp fused spur could be used.Submitted by: Electrical Safety Services

One would hope the manufacturer would supply a fitted cord that is suitable for the appliance, no matter what it's made of!!

Makes you wonder why any accessory manufacturer would produce a twin connection unit specifically to connect a hob and oven to the cooker control unit doesn't it?? ...lol!!

Fixed load appliances with suitably sized/fitted cords/leads ideally no more than 3 m in length, do not require a further thermal overload protective device such as a BS1362 fuse etc.

Go back and put these points to this guy, and see what he comes back with!! lol!!

 

[HappyHippyDad]

In Europe, only the UK has the facility of cord fuse protection via either a plug top or FCU. No-one ever seems to wonder how these same appliances (The exact same appliances will be available in the European markets, ...and beyond) are to be connected in those countries!!! lol!!

Seeing as both the cooking appliance you are referring too can be considered as fixed loads, (eg, ....cannot overload) the cords to these appliances cannot be stressed beyond their CCC due to overloading. The 32A MCB will quite happily look after any short circuit fault on the circuit!!

E54 could you explain this a little more please?

If we haven't been given a Zs how can we be sure that the 32A MCB will operate in the required time? A 32A type B needs ≥160A fault current to operate at 0.1s. So it needs a Zs of ≤1.44, wouldn't a quick Zs be a good idea?

 

[D Skelton]

E54 could you explain this a little more please?

If we haven't been given a Zs how can we be sure that the 32A MCB will operate in the required time? A 32A type B needs ≥160A fault current to operate at 0.1s. So it needs a Zs of ≤1.44, wouldn't a quick Zs be a good idea?

You're confusing meeting required disconnection times upon earth fault, with ensuring that the cable doesn't reach it's maximum operating temperature before the OCPD operates under short circuit and earth fault conditions.

Earth fault protection will no doubt be taken care of, overload protection is not needed because there is no chance of overload, and the provision of short circuit protection needs to meet no maximum disconnection time.

(k[SUP]2 [/SUP]x s[SUP]2[/SUP]) / I[SUP]2[/SUP] = t

 

[HappyHippyDad]

You're confusing meeting required disconnection times upon earth fault, with ensuring that the cable doesn't reach it's maximum operating temperature before the OCPD operates under short circuit and earth fault conditions.

Earth fault protection will no doubt be taken care of, overload protection is not needed because there is no chance of overload, and the provision of short circuit protection needs to meet no maximum disconnection time.

(k[SUP]2 [/SUP]x s[SUP]2[/SUP]) / I[SUP]2[/SUP] = t

Thanks DS,

Yes I was getting confused and it was actually reg 434.5.2 that I was thinking about. So if we look at the 1.5mm flex on the 32A MCB for example and we have a Zs of ≥1.44Ω then K²xS² ≤ I²xt so the conductor will exceed its permitted limiting temperature?

 

[D Skelton]

Thanks DS,

Yes I was getting confused and it was actually reg 434.5.2 that I was thinking about. So if we look at the 1.5mm flex on the 32A MCB for example and we have a Zs of ≥1.44Ω then K²xS² ≤ I²xt so the conductor will exceed its permitted limiting temperature?

No, that is only for faults of extremely short duration.

What you need to ensure is that the maximum operating temperature of the cable is not reached before the OCPD operates.

Lets assume that I[SUB]pf[/SUB] at the point of the reduction of the size of the cable is 0.5kA and work with an example:

(115[SUP]2[/SUP] x 1.5mm[SUP]2[/SUP]) / 500A[SUP]2[/SUP] = 0.12 seconds

The 32A b type circuit breaker will trip in 0.1 seconds if subjected to a fault current higher than 160A, in reality it will trip in 0.01 seconds, therefore it will trip long before the cable reaches its maximum operating temperature in 0.12 seconds.

 

[HappyHippyDad]

No, that is only for faults of extremely short duration.

What you need to ensure is that the maximum operating temperature of the cable is not reached before the OCPD operates.

Lets assume that I[SUB]pf[/SUB] at the point of the reduction of the size of the cable is 0.5kA and work with an example:

(115[SUP]2[/SUP] x 1.5mm[SUP]2[/SUP]) / 500A[SUP]2[/SUP] = 0.12 seconds

The 32A b type circuit breaker will trip in 0.1 seconds if subjected to a fault current higher than 160A, in reality it will trip in 0.01 seconds, therefore it will trip long before the cable reaches its maximum operating temperature in 0.12 seconds.

So what happens if subjected to a fault current less than 160A? i.e if Zs ≥1.44?

e.g: If we have a Zs of 1.5

K =115
S =1.5
I = 153

therefore (115² x 1.5²) / 153² = 1.27s

So at a fault current of 150A the 1.5mm flex will reach max temperature in 1.27s, but according Fig 3A4 the 32A MCB will act in around 15s?

 

[telectrix]

deleted due to miscalc.

 

[HappyHippyDad]

work it out. take a hypothetical Zs, work out the fault current, then apply adiabatic to calculate the trip time. e.g for a Zs of 2.3, I would be 100A with a trip time of 3 secs,

Sorry Tel, I was editing as you wrote with an example.

Also with your example wouldn't the live conductor reach its max temp in 2.8s and the MCB would take over 30s to trip?

 

[telectrix]

sorry, happy. i got a dec. point wrong.