Bonding water services

[Deleted member 26818]

Not quite there yet.
You need to find a time consistent with you measured value.
If for instance the suffent required for a device to operate in 5secs is 500A, but the current required to cause operatin in 1 sec is 750A and your measured value is 100A, then you would use the 750A and 1sec values.
If the measured value is greater than required to cause operatin in 0.4secs then you would use those values.
In the majority of cases, you will find that as the current increases, the required CSA decreases.

 

[zap]

Thanks again for your patience, Spin (and all).

How does this hyperthetical scenario look?

BS 88-3 100A

Measured:
889A
time 1.1
CSA 6.5mm[SUP]2[/SUP]

Tabulated:
time 5
580
CSA 9.1mm[SUP]2[/SUP]

Am I there yet?

I will get this understood by beer-o-clock tonight.

 

[zap]

Actually, I notice that the measured value has to be < 1 from the table so the csa would be slightly reduced.

 

[Deleted member 26818]

I'm not quite sure.
The table in appendix 3 for BS88-3 fuses indicates that a fault current of 889A would cause operation in less than 1sec.
1sec at 850A, so the values you would use are 850A and 1sec.
850 X 850 = 722500
722500 X 1 = 722500
√722500 = 850
850 ÷ 143 = 5.94mm².

 

[zap]

Thanks again, Spin.. I believe it's sunk in. One can teach an old ok dog new tricks, but it takes persistence!
My head is full now and it's beer-o-clock. I'm on my mobile now and don't know where the thanks button is so I'll do that tomorrow.

 

[SirKit Breaker]

Hi Guys,
I was going to use the following as a part of my assessment with niceic next month but I'm at a quandary.

The job was to swap a spurred socket outlet with a FCU and spurr off that to two double sockets, one next to the FCU in the hall and the other in the living room on the other side of the wall to which the first was installed.
I've upgraded the earthing conductor to a 16mm² and the main bonding conductors to 10mm² except for the water services.

I tested from a disconnected main bonding conductor at the MET to the water stop cock valve body and the highest measurement attained was 0.04 ohms. This figure seems too low (6mm² and at least 10m long possibly more) and leads me to believe a parallel earth path exists. Also the conductor has green (not g/y) insulation and is 6mm² (should be 10mm²).

Here's my issue: The kitchen has been fitted impressively. I mean all the pipes etc. were jointed after the units were installed. Perfectly formed, circular holes in the back of the sink cupboard. I've never seen such impressive workmanship.
I know, now, that I shouldn't have installed the extra sockets until the earthing arrangements were upgraded but that's history.
The owner of the property is not happy for me to start destroying his lovely kitchen and I'm reluctant to do so. Can I record a limitation?

I'll be very grateful if any of you can help.

You may be getting your knickers in a knot about nothing, do the adiabatic equation, and your "undersize bonding cables" may be OK, and if you can show the assessor your calculations, hey presto, no problems.........

Cheers.............Howard

 

[rattlehead85]

This subject arose a couple of weeks back with some great posts and very interesting theories. I to had misunderstood BS7671 regarding earthing and bonding minimum sizing for years.
This week i had a job to carry out some alterations to a kitchen circuit which therefore required evaluating the suitability of the existing arrangements.
The main supply was a TN-S wired in 10mm2 T&E with the earthing conductor being the 4mm2 within the cable. The main fuse was a 1361 40A with a Ze of 0.21 and PSC of 1.2KA.
The existing bonding was 6mm2 to both services.
Applying the adiabatic equation using the tables for a 1361 40a fuse i confirmed the existing 4mm2 earth employed was greater than the minimum calculated size therefore suitable for ongoing use and also confirming the bonding could in fact be left. Previously i would have asked for the earthing to be upgraded to 16mm2 and bonds to 10mm2 as a given.
Would be interested to see how many sparks / inspectors would have condemned the 4mm2 earthing conductor just because 16mm2 wasn't in-situ.

 

[zap]

Thanks once again all contributors. I think if anyone else is finding 'sizing of protective conductors' difficult to understand this could be the thread for them. Like most things, it's easy when you know how.

 

[Engineer54]

I'm always a little confused about these earthing and Bonding cable sizing questions, that seem to be appearing on a regular weekly basis now. ....Don't they teach anything any more at collage, or anything about when, where and how to use adiabatic equations?? With some of the calculations being put forward on these threads, ...it seems Not!!

 

[rattlehead85]

I'm always a little confused about these earthing and Bonding cable sizing questions, that seem to be appearing on a regular weekly basis now. ....Don't they teach anything any more at collage, or anything about when, where and how to use adiabatic equations?? With some of the calculations being put forward on these threads, ...it seems Not!!

Well i agree but is it any wonder with all these nonsense Electrical Trainee courses being banded about. Adiabatic equations and calcs are not something you learn overnight and with what they try to cram in in that short space of time i doubt the tutors even touch the surface of it.....that's if the tutors actually know themselves. Some of the stories i've heard are pretty bad. Perhaps that is why some of the part p work is so shoddy. You probably would learn a far greater amount on this forum from proper time served sparks and knowledgeable people than you would attending one of them money thieving time wasting comedy acts.