4mm earth off supply cable with bonding clamp

J

J.C.E

~

Arms

Dec 2, 2010

662

165

118

Earth

• Dec 17, 2016

• #1

Afternoon guys
done the changeover of the c/u's yesterday on a rewire I done of a house refurb.

The supply cable just had a bonding clamp with a 4mm g/y single into a MET.

So I have just put my 16mm main earth into the MET.

Am I good to just stick on my EIC something along the lines of 'strongly recommend suppliers earthing is upgraded and use of correct banding rather than bonding clamp' and then advise the customer (in this case it is the builder/developer- who is then just selling in) to contact the DNO to do this.

for your information- the clamp was tight and ze was 0.18 (tns)

 

T

The Ghost

• Dec 17, 2016

• #2

What does adiabatic say? And generally yes I would recommend DNO supplying proper sweated on earth with 16mm² btw I get 3.5mm²

 

L

Leesparkykent

• Dec 17, 2016

• #3

Based on a 100A bs1361 fuse

I=1277.77
t=0.4
k=143

230/0.18= 1277.77

1277.77²=1632696

1632696x0.4=653078

square root of 653078=808

808/143=5.65mm

 

J

J.C.E

~

Arms

Dec 2, 2010

662

165

118

Earth

• Dec 17, 2016

• #4

thanks lee- it is a 100a main head- but its 4mm from sheath to met

 

L

Leesparkykent

• Dec 17, 2016

• #5

J.C.E said:

thanks lee- it is a 100a main head- but its 4mm from sheath to met

Click to expand...

It needs upgrading. Is it def a bs951 clamp? some older installations it looks like a bs951 clamp but its actually shaped/grooved to accommodate the cable. If it is then you could just replace the 4mm and put a safety electrical connection do not remove tag on the cable. If not you could make a call to the DNO and they will prob come out for free and put a Hepworth clamp on it.

 

telectrix

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Telectrix

• Dec 17, 2016

• #6

Leesparkykent said:

Based on a 100A bs1361 fuse

I=1277.77
t=0.4
k=143

230/0.18= 1277.77

1277.77²=1632696

1632696x0.4=653078

square root of 653078=808

808/143=5.65mm

Click to expand...

not got figures for 1361,s but a BS88 will go in 0.2sec. @1277A

 

• 

Reactions: Leesparkykent

L

Leesparkykent

• Dec 17, 2016

• #7

Got it out my brown 16th

 

T

The Ghost

• Dec 17, 2016

• #8

Mhm interesting, I based on the downstream fuse ie 60898. I am happy to be corrected. But why do you base it on the upstream fuse just looking to learn something here.

 

T

The Ghost

• Dec 17, 2016

• #9

@Leesparkykent Sorry the above does not really sound coherent, should have said I based my adiabatic on... As well I had K as 115 based on 6242y. Any reasoning for your figures appreciated. And of course my latest Amd 3 does not have 1361 curve in it

 

• 

Reactions: Leesparkykent

W

Wilko

• Dec 17, 2016

• #10

Vortigern said:

@Leesparkykent And of course my latest Amd 3 does not have 1361 curve in it

Click to expand...

Here's one. Mind you, I can't see to read any sense into it tonight

 

• 

Reactions: Vortigern and Leesparkykent

L

Leesparkykent

• Dec 17, 2016

• #11

544.1.1 comes in to play here where it states the earthing conductor for the installation shall not have CSA of less than 6mm. 132.6 The CSA of conductors shall be determined for both normal operating condition and, where appropriate, for fault conditions.

 

W

Wilko

• Dec 17, 2016

• #12

100A fuse is supported by a DNO supplied earth of 4mm - can't ever have been right can it? Maybe there's 60A fuse in the cutout and it's a 6mm earth cable. Apologies - not meaning to doubt J, just saying ouch.

 

T

The Ghost

• Dec 17, 2016

• #13

@Leesparkykent thanks for that will look into it and get back to you. It worries me that I am thinking downstream and you upstream. I can explain my reasoning for that (probably wrong) but I feel I need to get to the bottom of what is right side up here. Will look into regs quoted and try to work that out.
@Wilko A1 thanks for that nice one. No 4mm² by regs is not right which I would follow and put in 16mm², overkill? but why not, belt and braces is a good approach.

 

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Reactions: Leesparkykent

L

Leesparkykent

• Dec 17, 2016

• #14

K is selected from table 54.2 as the main earthing conductor is insulated, not incorporated in a cable and not bunched.
To comply with 542.1.3.1 the earthing arrangement shall be such that earth fault currents and protective conductor currents which may occur are carried without danger, particularly from thermal, thermomechanical and electromechanical stresses. Dividing voltage by Ze gives us the prospective earth fault current to base our calculations on.

 

Last edited by a moderator: Dec 17, 2016

telectrix

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Telectrix

• Dec 18, 2016

• #15

Leesparkykent said:

544.1.1 comes in to play here where it states the earthing conductor for the installation shall not have CSA of less than 6mm. 132.6 The CSA of conductors shall be determined for both normal operating condition and, where appropriate, for fault conditions.

Click to expand...

BS7671 does not apply as the conductor in question is DNO equipment and thus outside the scope of BS7671.

 

T

The Ghost

• Dec 18, 2016

• #16

@Leesparkykent So looking through the regs you quote, clearly cant dispute anything there. But still back to my original question why are you using the main fuse for your calculation? My thinking is that a fault occuring will be from the installation itself rather than the DNO equipment. And anyway I am surely only looking at the installation after the DNO equipment. It is important to get this clear in my mind for obvious reasons. So again looking for the reasoning behind using DNO equipment for calculations (thats their business aint it) Sorry to be a pest, and perhaps ignorant but .....I am

 

T

The Ghost

• Dec 18, 2016

• #17

Deleted

 

W

Wilko

• Dec 18, 2016

• #18

My thought was to use the service fuse for calcs as it's the worst case. A tail shorted to earth in the metal CU will see all electrons fleeing through that wee cable, I think.

 

T

The Ghost

• Dec 18, 2016

• #19

@Wilko Yeh I can see that and test is required at origin of installation, and of course we want the biggest "bang" possible in calcs. But you can see I based my calcs (mistakenly?) downstream and Leesparkykent upstream the difference being me thinking 4mm² is ok and Lee proving its not. Quite a serious difference which worries me. I know I'm being like a dog with a bone but I beleive its important to understand this from a regs and science viewpoint. And also what I was taught and what seems to be reality have a dangerous divergence potentially. Unless I misunderstood what I was taught on C&G course (entirely possible) anyway I will shut up now and mention it no more....

 

L

Leesparkykent

• Dec 18, 2016

• #20

It would depend where the fault was in a circuit/circuits. Closer to the origin the higher levels of fault current hence sizing the main earthing conductor to the worst possible scenario based on Ze. The fault current/level diminishes with impedance.