Cooker circuit fault blew DNO fuse !

[Revved Up Sparky]

I was called out to a job on Wednesday (in an apartment) where a DIY electrician had wired a cooker switch incorrectly, he'd put both lives in the 'supply L & N' terminals and both neutrals in the 'load L&N' terminals ! which resulted in a L-N short when he switched it on !.

However instead of blowing the 30A BS3036 rewireable fuse in the Wylex fuseboard or the 60A BS1361 in the submain switchfuse, it took out the DNO fuse which was in a sealed panel. My first thought was that the fusewire in the 30A fuse had been replaced with copper wire etc but that was not the case. One thing I did observe was that his apartment was the closest to the DNO panel where the fuse is.... so perhaps the fault current having a shorter path could cause this ?.

My advice to the DIY chap, other than 'get a proper spark in next time' was to get MCB's fitted instead of rewireables which should hopefully trip before the DNO fuse blows. One thing I was unable to do (because the power was off) was check Zs. All I did was connect the switch correctly and tell the owner to phone the DNO to get the fuse replaced.

Any thoughts anyone ? what do you think could be causing this discrimination issue ?..... thanks in advance

Happy Easter

 

[matt.leung]

OMG

 

[sparkytim]

Common experiance, operating time for a 60 Amp BS1361 faster then a 30 Amp 3036 when dead short causing very high fault current.

 

[Revved Up Sparky]

Common experiance, operating time for a 60 Amp BS1361 faster then a 30 Amp 3036 when dead short causing very high fault current.

Cheers Tim, that's interesting. I've been a spark for 20 years and not had this scenario very often hence my question. I have'nt done any research on this yet so I'll throw out the question ; If there had been a 60898 32A MCB protecting the cooker circuit, would the MCB have activated first - even with such high fault current ?.

 

[Evans Electric]

Discrimination was more of an issue years back, when the usual situation was the old Wylex rewirable fuses backed up with the 60/100A HRC supply fuse . Quite common for a fault to by-pass the rewirables and take out the supply HRC .

It used to be called "Coarse" or "Close" protection by the way , coarse for fusewire , close for HRC or MCB.

 

[sparkytim]

Cheers Tim, that's interesting. I've been a spark for 20 years and not had this scenario very often hence my question. I have'nt done any research on this yet so I'll throw out the question ; If there had been a 60898 32A MCB protecting the cooker circuit, would the MCB have activated first - even with such high fault current ?.

Probably it would

Its all in the curves BRB APPENDIX 3

Note Pg 245 BS 3036 Fuses 30 A will need 450Amps to blow in 0.1 sec

Compare with Pg 249 60898 MCBs 32 A will need only 160 Amps.

If you do a L-N PFC test at the Cooker switch you will see what the Fault current could well have been, Possible 1KA or more if Zs low enough.

The Service Fuse was doing its job in providing Back-up protection.

 

[SPECIAL LOCATION]

Cheers Tim, that's interesting. I've been a spark for 20 years and not had this scenario very often hence my question. I have'nt done any research on this yet so I'll throw out the question ; If there had been a 60898 32A MCB protecting the cooker circuit, would the MCB have activated first - even with such high fault current ?.

I once switched on a socket circuit that I had previously put in a temp strap L-E whilst doing some investigation into some broken wires..

That was a L-E, TNS short.. not quite a L-N, but it only took the 32A 60898 & NOT the DNO's fuse!

:C

 

[NozSpark]

I've had this befor as well,,, got me confused for a bit!

 

[PC Electrics]

Doesn't really surprise me. 3036s ar just too slow compared to 1361s or 60898s. Might just as well have nails in there. It's one of the reasons I give to people for changing CUs.

 

[Andy™]

dont forget the service fuse will already have the load of the rest of the house. the fuse to cooker wont

seen a dead short L-E on a ring main take out a 60A 1361 before 32A 60898. shower & cooker were in use at the time, so fuse was probably already warm

 

[Revved Up Sparky]

Thanks for all the informative replies which are much appreciated. I have certainly learnt something today ..... and good point Andy

 

[kerching]

Brother in law did exactly the same....I left him a red and a brown in one connector and blue/black in the other. All he had to do was take connector off and put wires into back of socket. He decided that red&black and brown & blue was the way forward. Put fuse back in [3036] with board still switched on...nearly blew him out through his wedding ring. I loaned him my big FO rechargeable lantern torch until DNO arrived to replace service fuse. Mr DNO was so impressed with Big FO lantern torch that we gave it to him in exchange for a '5 minutes root in the back of his van' [ not in a Mr Glitter sort of way, mind you!!]. Replenished my stock of fuses and other sundry items. Win WIn all round... :coat

 

[steptoe]

Brother in law did exactly the same....I left him a red and a brown in one connector and blue/black in the other. All he had to do was take connector off and put wires into back of socket. He decided that red&black and brown & blue was the way forward. Put fuse back in [3036] with board still switched on...nearly blew him out through his wedding ring. I loaned him my big FO rechargeable lantern torch until DNO arrived to replace service fuse. Mr DNO was so imoressed with Big FO lantern torch that we gave it to him in exchange for a '5 minutes root in the back of his van' [ not in a Mr Glitter sort of way, mind you!!]. Replenished my stock of fuses and other sundry items. Win WIn all round... :coat

had a sh** day at work but this has made me chuckle,

thanks.

 

[markysparky]

Was drilling some trunking for a replacement leg of a damaged kitchen circuit, and drilled with my first hole straight into the cooker cable. Massive flash around the trunking!! The cables were not run in the correct zones so had no idea cables were there.

Also this took out the main 60A fuse leaving the 30A 3036 intact. Added at least another hour onto my day as had to chase the wall to repair the cable. Luckily had a spare fuse on the van.

 

[batty]

I chopped through a ring main cable which was run along top of skirting it didn't take main fuse out and it didn't blow rewirable fuse just blew the bolster out my hand lucky I survived.

 

[Andy™]

I chopped through a ring main cable which was run along top of skirting it didn't take main fuse out and it didn't blow rewirable fuse just blew the bolster out my hand lucky I survived.

came accross that many times, dead short and the fault has blown its-self clear before the 3036 has

 

[Megaohm7]

Doesn't really surprise me. 3036s ar just too slow compared to 1361s or 60898s. Might just as well have nails in there. It's one of the reasons I give to people for changing CUs.

You've seen nails used in rewirables too have you ; -)