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

 

[D Skelton]

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?

Then in this scenario, with an MCB as opposed to a fuse, and a 1.5mm cable, you have a problem. Zs being exceeded however doesn't neccesarily mean that the cable's maximum operating temperature will be exceeded too under fault conditions.

 

[Engineer54]

Then in this scenario, with an MCB as opposed to a fuse, and a 1.5mm cable, you have a problem. Zs being exceeded however doesn't neccesarily mean that the cable's maximum operating temperature will be exceeded too under fault conditions.

I'd go as far as saying, ''UNLIKELY'!!

Another reason why you should never be happy with only achieving the max Zs values when designing circuits/installations!! Then again we DO have our cure all /get out of jail card RCD devices!! lol!!

 

[D Skelton]

I'd go as far as saying, ''UNLIKELY'!!Another reason why you should never be happy with only achieving the max Zs values when designing circuits/installations!! Then again we DO have our cure all /get out of jail card RCD devices!! lol!!

Ah yes, those magical devices that cure all problems! Haha

 

[HappyHippyDad]

I'd go as far as saying, ''UNLIKELY'!!

Another reason why you should never be happy with only achieving the max Zs values when designing circuits/installations!! Then again we DO have our cure all /get out of jail card RCD devices!! lol!!

With most TT's I guess you're relying on the rcd for fault protection though? I'm not talking about good or bad practice, just the fact that most will have Ra's too high to aid in disconnection solely from the MCB or fuse.

Before the advent of RCD's I'm assuming far more care was taken with regards supplementary bonding and cable selection?

 

[MJS9691]

Thanks for the replies everyone I see that "greyness" was thrashed out in the end;

So to conclude, we're good to go into the dual plate but will install a fire extinguisher next to the oven just in case?:rofl:

 

[Engineer54]

With most TT's I guess you're relying on the rcd for fault protection though? I'm not talking about good or bad practice, just the fact that most will have Ra's too high to aid in disconnection solely from the MCB or fuse.

Before the advent of RCD's I'm assuming far more care was taken with regards supplementary bonding and cable selection?

I can take you to more than just a few Domestic TT systems in the Essex area, that have Ra values better than some local DNO TN-S Ze values and no the gardens aren't full of copper rods either. lol!

One thing about the older TT systems, the electricians of the time understood the how's, why's and where's, of TT'ing and they didn't use thin twigs bashed in the ground right next to a wall!! ...lol!!

 

[nickblake]

All im going to say on the matter is you are finding 8.5 kw hobs wired in non tri rated 2.5mm flex protected by 32 amp mcb's way over loaded if all the hobs are on in my eyes its wrong should be a minimum of 4mm but who am i ill wire it up if it goes wrong it was installed in accordance to the manufacturers instructions there problem not mine