Disconnection times

[diditrain]

Hi all, can someone tell me what the disconnection time is for circuits supplied from a distribution circuit?

 

[johnboy6083]

Final ccts are 0.4 secs, distribution ccts are upto 5 secs. This is to allow for discrimination on large installations.

 

[diditrain]

So circuits supplied from a distribution circuit, not a final circuit are 5 secs?

 

[MDJ]

fixed equipment is 5 seconds, portable 0.4 usually.

 

[malcolmsanford]

Have a look at Table 41.1 and then have a read of regs 411.3.2.2 through to 411.3.2.6

 

[diditrain]

Ok lets not get bogged down with this, maybe i am thinking about it too deeply.
If i have a distribution circuit then the disconnection time is five seconds. Are the circuits supplied from this distribution circuit then classed as a final circuits and then subject to the 0.4 second disconnection time provided they are protected by a device of 32 amps or less.

 

[malcolmsanford]

And if it is a TT system ?

Have a read of the sections I said and see what you think.

 

[IQ Electrical]

fixed equipment is 5 seconds, portable 0.4 usually.

Not for the last 3 years.....

 

[Sparky Ninja]

As malcolm has stated, you need to know the earthing system first.

 

[Sparky Ninja]

Not for the last 3 years.....

protective conductor currents on a lounge radial and now this...

.. sometimes on a monday morning all you can do is. :mad2:

 

[adslev]

Right well I'm having arguments at work with a fellow' tester'Basically its an old installation 40+ all bs88 fuses, its a tn system so 32 and under should be 0.4 is my view however he argues there all 5s because a bs88 fuse cannot achieve a 0.4 disconnection time without the installation being protected by a trip, can't find much in the regs because the install is so old

 

[malcolmsanford]

Well according to appendix 3 they expect a BS 88-2.2 and an 88-6 to trip out in anything from 0.1 to 5 secs, and I don't think that as changed much throughout the years.

What is a trip?? an MCB ??

 

[JUD]

Right well I'm having arguments at work with a fellow' tester'Basically its an old installation 40+ all bs88 fuses, its a tn system so 32 and under should be 0.4 is my view however he argues there all 5s because a bs88 fuse cannot achieve a 0.4 disconnection time without the installation being protected by a trip, can't find much in the regs because the install is so old

Of course they can achieve 0.4s if the fault current is high enough. Have a look at Table 41.2 BRB for Max Zs values for 0.4s.

Basically...

TN system

Final circuits ≤ 32A = 0.4 seconds
Final circuits > 32A and distribution circuits = 5 seconds

TT system

Final circuits ≤ 32A = 0.2 seconds
Final circuits > 32A and distribution circuits = 1 second

 

[IQ Electrical]

Right well I'm having arguments at work with a fellow' tester'Basically its an old installation 40+ all bs88 fuses, its a tn system so 32 and under should be 0.4 is my view however he argues there all 5s because a bs88 fuse cannot achieve a 0.4 disconnection time without the installation being protected by a trip, can't find much in the regs because the install is so old

As fellow 'testers' I'm a bit concerned that neither of you could access the facts without the help of a forum?

It's pretty basic stuff....

 

[adslev]

Yes well my copy of bs7671 wasn't available to show him the regs, iv only got into testing full time recently and the other tester has been doing it for 30+ years and wouldn't belive me. I have quite a few doubts about him but hes been the main/only tester the company I work has ever had and does not take kindly to me questioning him. But thanks for you knowlegable responses

 

[IQ Electrical]

It was the bit where you said 'can't find much in the reg's...'

What has the age of the installation got to do with the disconnection time of a fuse to BS 88?

 

[adslev]

Well when i told him about tn systems having a 0.4 32amp and under and 5sec diconnection time his arguement was that because the installation was so old that he would test it to the 15th edition of the regs, like i said im not going to argue because I will not win. I am used to doing eic reports with other firms I have worked with not pir's. I only gained my 2391-10 in May of this year and im only 24 so I hoping to learn aswell but as I said he's set in his ways and will not back down