Bonding water services

[zap]

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.

 

[SJD]

What would you expect the resistance to be for 10m of 6mm² single core cable (e.g. from the OSG tables)?

 

[Plonker 3]

Why not bond it at the nearest accessible point and note it on the certificate?

 

[i=p/u]

Would you bond the gas if entres gas box in plastic and then copper to combi???

 

[bigbaddave]

Why not bond it at the nearest accessible point and note it on the certificate?

Why bother if the 6mm is ok? I think the op point is it installing it that is going to be pig of a job

What are the earthing arrangements?

 

[Plonker 3]

Why bother if the 6mm is ok? I think the op point is it installing it that is going to be pig of a job

I agree but as the OP says it is for a assessment, just for the sake of a few meters of cable it would be worth doing especially as he has already renewed the gas bond. Also the assesses can sometime be picky, but the maybe he could use a few regs numbers to back his argument up on why he didn't upgrade it.

 

[bigbaddave]

They can be as picky as the want but if it complies as per the regs then if it not reasonably practical then I wouldn't be doing it. As above though bE prepared to have some evidence for things you have noted on the cert .

However what earthing system is it?

 

[Plonker 3]

Picky?

Sometimes they think they write the bloody regs and amend as they see fit.

 

[bigbaddave]

Aype.. I had that on my first one. gave him the cert for the job and used the printable ones off IET site.. Anyway first think he said was ahh these dont comply... Really, No there not in the correct format.. Anyway page whatever of the regs and guess what the identical form.. That shut him up

 

[zap]

Thanks for all your suggestions. I have several balls in the air at the moment and unable to sit in front of my computer.
It's a tn-s system.
I haven't checked the conductor resistance yet but I will a bit later when I've dropped some of the balls.

 

[zap]

Aype.. I had that on my first one. gave him the cert for the job and used the printable ones off IET site.. Anyway first think he said was ahh these dont comply... Really, No there not in the correct format.. Anyway page whatever of the regs and guess what the identical form.. That shut him up

Interresting! I have not used the niceic forms, I've used the downloadable forms from the iet which I believe are replicas of those in the bgb.

Back in a while...

 

[zap]

What would you expect the resistance to be for 10m of 6mm² single core cable (e.g. from the OSG tables)?

0.03 ohms - took my eye off the ball and manage to double it before!

... that solved that bit of the equation; moving on...

The gas and power are supplied in a garage. The water, according to the owner, is at the other end of the house.
There's a boiler in the garage and all the pipes disappear upwards. The existing water bonding conductor goes into plastic trunking and it too disappears upwards (not in the same place).

If I could replace the existing conductor without damaging anything I would. But if there's a less destructive solution, all the better for all interested parties.

 

[Rampantchilli]

Picky?

Sometimes they think they write the bloody regs and amend as they see fit.

ive had 2 of my jobs assessed by someone who DID write the regs, his name is in the list of contributors

 

[Rampantchilli]

. The water, according to the owner, is at the other end of the house.
There's a boiler in the garage and all the pipes disappear upwards. The existing water bonding conductor goes into plastic trunking and it too disappears upwards (not in the same place).

If I could replace the existing conductor without damaging anything I would. But if there's a less destructive solution, all the better for all interested parties.

why not bond across all of your boiler pipes, including water, make a note on the cert

 

[D Skelton]

If I could replace the existing conductor without damaging anything I would. But if there's a less destructive solution, all the better for all interested parties.

Why haven't you carried out an adiabatic equation? Chances are the csa of the water bond is fine as is.

 

[keniff]

Isn't adiabatic for main earthing conductors and cpc? Didn't think you could use that for bonding?

 

[D Skelton]

Correct, so on a TN-S system to size a bonding conductor the main earthing conductor must be sized correctly. The OP said he just whacked in some 16mm but I would bet he gave this very little thought. The amount of times I've actually put in a 16mm MEC in a domestic situation because of requirement I could probably count on one hand.

Chances are, the original 10mm MEC was absolutely fine. If so, he could leave the 6mm main bond in place.

 

[zap]

Correct, so on a TN-S system to size a bonding conductor the main earthing conductor must be sized correctly. The OP said he just whacked in some 16mm but I would bet he gave this very little thought. The amount of times I've actually put in a 16mm MEC in a domestic situation because of requirement I could probably count on one hand.

Chances are, the original 10mm MEC was absolutely fine. If so, he could leave the 6mm main bond in place.

You're absolutely correct (and you win the bet), I didn't think too much about installing 16mm² for the MEC because I thought I had to! Have I misunderstood something somewhere?
I'm going to read chapter 54 in the bgb again. I've lost count how many times I've read it, and unfortuneatly the misunderstood is sticking.

 

[D Skelton]

In short the MEC is a protective conductor, therefore it can be sized using an adiabatic equation. The bonding conductors can then be sized as not less than half the REQUIRED size of the MEC. TN-S system that is. Ergo sum, if the required size of the MEC is less than 10mm, then your main bonding conductors can be 6mm.

 

[zap]

why not bond across all of your boiler pipes, including water, make a note on the cert

This would be relatively simple to achieve. Thanks for the suggestion. Now back to ch 54 for the meanwhile.

 

[zap]

For those reading bgb 542.2.3 - Note: ... BS7430

 

[polo1]

Err..... 542.3 :angel_smile:

 

[zap]

Err..... 542.3 :angel_smile:

click the link

 

[zap]

Isn't adiabatic for main earthing conductors and cpc? Didn't think you could use that for bonding?

bgb 543.1.1 ... , other than a protective bonding conductor, ...

supports keniff's post, unless I've misunderstood, in which case please clarify.

 

[telectrix]

calculate the min. size required for main earth using adiabatic equation. then the bonding needs to be > or = to 50% of that

 

[zap]

calculate the min. size required for main earth using adiabatic equation. then the bonding needs to be > or = to 50% of that

Z[SUB]s[/SUB] = 0.26 (at cu - included to verify voltage)

I[SUB]pf [/SUB]= 889 (measured)
t = 5 (main protective device > 32A)
k = 143 (from bgb table 54.2)

therefore:

S = (SQR (889[SUP]2[/SUP] x 5)) ÷ 143

= 13.9mm[SUP]2[/SUP]
Nearest larger conductor csa = 16mm[SUP]2[/SUP] and the bonding conductors 10mm[SUP]2[/SUP] (such that 10 >= 16 ÷ 2)

If my calculations are correct I've upgraded the cable correctly and now understand why! Please let me know if I've miscalculated.

I like Rampantchilli's solution below and bgb 544.1.2 applies if it's impracticable to connect within 600mm of the incoming water service. The property doesn't have a water meter.

Will this satisfy the niceic inspector?

Originally Posted by Rampantchilli
why not bond across all of your boiler pipes, including water, make a note on the cert



Read more: http://www.electriciansforums.net/newreply.php?do=newreply&p=696945#ixzz2Izwpi1Ap​

 

[johnboy6083]

Use the time from the time/current graphs in the appendix. This will make your calc more accurate.

 

[Deleted member 26818]

Thanks for all your suggestions. I have several balls in the air at the moment and unable to sit in front of my computer.
It's a tn-s system.
I haven't checked the conductor resistance yet but I will a bit later when I've dropped some of the balls.

As far as I'm aware, the minimum CSA for bonding conductors is 6mm² for TN-S earthing systems.
TN-C-S systems require 10mm².

When you conduct the adiabatic equation, you either use measured values and the disconnection times for those measured values.
Or use those from the tables in appendix 3, not a mish mosh of the two.
As far as I can see, the current required for a 100A BS1361 fuse to operate within 5 secs is 630A, 889A would cause operation within about 1.5secs.
As you haven't supplied the information as to the rating or type of device, a BS1361 100A fuse is most likely the worst case scenario.
Either use the tabulated value of 630A along with the time 5secs, or use the measured value of 889A and the time of 1.5secs.

Using 889A and 1.5secs will result in an earth conductor CSA of 7.6mm².
Using 630A and 5secs will result in a CSA of 9.85mm².
Bonding conductors are required to be half of the CSA of that required for the earth conductor.
Either half of 7.6mm² = 3.8mm², or
half of 9.85mm² = 4.925mm².
In any event 6mm² is more than sufficient.

 

[zap]

Use the time from the time/current graphs in the appendix. This will make your calc more accurate.

This looks like the kiddie. I've got the csa of the mec to < 10mm[SUP]2[/SUP] so a 6mm[SUP]2[/SUP] bonding conductor should be fine.

Please sanity check my method. If it's right it's stuck and shouldn't give me any problems in the future. If it's not right, hopefully someone will correct me. Almost there:6:

Select Overcurrent Protective Device from bgb App3 e.g. Type B
Scan the 5 second line until it crosses the fuse rating curve e.g. 100A
Read from bottom e.g. 500A
Perform adiabatic equation using this value

(SQR(500[SUP]2[/SUP] x 5)) ÷ 143 = 7.8mm[SUP]2[/SUP]

This is hyperthetical as I don't have the relevant information at hand.

 

[zap]

As far as I'm aware, the minimum CSA for bonding conductors is 6mm² for TN-S earthing systems.
TN-C-S systems require 10mm².

When you conduct the adiabatic equation, you either use measured values and the disconnection times for those measured values.
Or use those from the tables in appendix 3, not a mish mosh of the two.
As far as I can see, the current required for a 100A BS1361 fuse to operate within 5 secs is 630A, 889A would cause operation within about 1.5secs.
As you haven't supplied the information as to the rating or type of device, a BS1361 100A fuse is most likely the worst case scenario.
Either use the tabulated value of 630A along with the time 5secs, or use the measured value of 889A and the time of 1.5secs.

Using 889A and 1.5secs will result in an earth conductor CSA of 7.6mm².
Using 630A and 5secs will result in a CSA of 9.85mm².
Bonding conductors are required to be half of the CSA of that required for the earth conductor.
Either half of 7.6mm² = 3.8mm², or
half of 9.85mm² = 4.925mm².
In any event 6mm² is more than sufficient.

Thanks Spin, ships in the night spring to mind. Looking at your logic I understand and I believe my hyperthetical workings are similar to yours. Please let me know if you disagree.

If I've finally got it I'd like to thank all contributors to the thread and hope I can provide assistance to others in the future.

 

[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.