Breaker not tripping

[bensk]

Hi Everyone,

I have been asked to install a cooker and hob built in one (total load 15kw ish), the customer has already 6mm installed from previous cooker so i changed the breaker to 32A b type (I hope this is okay?) and explained that it might trip if he is using the whole oven and all hobs which he is fine with. However we decided to test the breaker out so once installed we turned all hobs on and cooker but the breaker does not trip?? I was confused by this as I expected the breaker to trip, so I measured with my amp clamp and it was showing 47a going through and still not trip?? Can someone please explain or comment if you know why the breaker is not tripping as it's of big concern to me.

Thanks for reading.

 

[kingeri]

MCBs will not immediately trip when their rated current is exceeded. A 32 amp B-Type will take 47 amps for quite some time before tripping. Its all in BS7671, unfortunately it's too early for me to put my cornflakes down and give you the page numbers!

 

[stevethesparks]

BYB page 325. Approximately about 40 minutes. However, the 47 Amps wouldn't have been constant due to cycling of the cookers elements.

 

[Simonslimline]

Edited post.

 

[Simonslimline]

I would be more concerend as to why they got yourself to carry out this work. No offence intended but it seems perfectly clear that you are out of your depth. Electricity is not guess work and is lethal.


EDIT- I see you are an improver. Maybe get access to the trainee section. Lots of good helpful people in there.

 

[telectrix]

@ OP. what was the previous MCB rating? how is the 6mm cable installed. i.e.ref. method?

 

[bensk]

Hi it was 40A type B and about 5m under floor boards and then chased in wall to worktop height for the cooker isolator switch. I didnt know the MCB could take upto 45 mins before tripping?! could not this be enough time to damage the cable??

Thanks again for replies!

 

[bensk]

@ OP. what was the previous MCB rating? how is the 6mm cable installed. i.e.ref. method?

40A B type, and the 6mm cable is under floor boards for about 6m and then chased in wall upto work top height to the cooker isolator switch.

So if the breaker can have upto 47 amps going through it for 40 minutes without tripping isn't that enough time for the cable to be damaged?!


Thanks for your replies too.

 

[telectrix]

no, the cable rating allows for short term current in excess of it's rating. e.g. if BS7671 gives a value of 45A (6mm cable) then that value assumes constant load of 45A. 20% -30% above that for short periods is within the limits of the cable. obviously, if cable derating factors are applied, the current capacity will be lower., but a short term excess is allowed for . so you can see from MCB curves in app. 3 of 7671, that a MCB will withstand 1.45 x In for about 1 hour. this also means that the cable will handle it as well.

 

[Midwest]

Whole chapter on it in BS7671, chapter 43, don't ask me to explain it all, though.

By the way, think 40amp mcb for 6mm cable (depanding on exactly what type of cable it is), is somewhat pushing the design envelope.

Found this little discussion; IET Forums - help with understanding cable overload calculation please

and a JW video; https://www.youtube.com/watch?v=bBYHtBPbBJE

 

[hightower]

OP your profile has you listed as level 3 trained - how've you managed to pass a level 3 qualification being oblivious to how devices operate, pretty sure this is covered at level 2

 

[Midwest]

OP your profile has you listed as level 3 trained - how've you managed to pass a level 3 qualification being oblivious to how devices operate, pretty sure this is covered at level 2

It would depend on his age. If it was anything like my training at 16, I was more interested in the evils of the flesh, than protection against overcurrent. Shame I couldn't revisit my training, I would be a lot more riveted, if you excuse the pun!

 

[Davisonp]

BYB page 325. Approximately about 40 minutes. However, the 47 Amps wouldn't have been constant due to cycling of the cookers elements.

If you saw my post before editing, I was wrong. I got the wrong lines in the diagram and my label for 40 Amps is in fact 60 Amps, it looks like the original 40 minutes is correct.

 

[hightower]

Have I got it wrong?
The time/current chart for B type 60898 breakers says the 32A MCB should trip at roughly 100 seconds for 50Amps, or 200 seconds for 40 Amps. That makes it a maximum trip time of 4 minutes, not 40.

View attachment 32130

You're reading the graph wrong me thinks

It would take 70a to trip a 32a breaker in 100 seconds

 

[Davisonp]

You're reading the graph wrong me thinks

It would take 70a to trip a 32a breaker in 100 seconds

Spotted that as soon as I posted it, I was looking at the line beyond the 40A MCB curve as being 40 Amps, where it is actually the 60 Amp line.