Earthing a metal shed

brman · Dec 4, 2012

spinlondon said:
No the ratio of conductivity between copper an steel is about 8:1.
Copper is eight times more conductive than steel.
As such for the SWA to have copper equivelance, the CSA of the armour would have to be in the region of 80mm².
With 2core 70° C PVC cable the minimum CSA of the conductors would be 70mm², 3core, 50mm², 4core, 25mm².
Of course, if you are using conductors with those CSAs, the chances are that you would require a larger than 10mm² bonding conductor.

Ah, Thanks! I thought the figures in that table were copper equivelent CSAs, not actual. That will teach me to skim read things!

The table I have used before is this one.... http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf
It shows the copper equivalent CSAs although not the 8:1 you quote. Using this table it does show you can get away with the swa as bonding in some sizes, not really with PME though......

Deleted member 26818 · Dec 4, 2012

The earthingnuts chart is based on the same information as John Peckham's chart.
The main difference being, that John has conducted the adiabatic equation for each of the cables, rather than just relied on Table 54G/54.7.
The earthingnuts chart does not refer to the ratio of conductivity between copper and steel.
All the earthingnuts chart indicates, is the CSA required to satisfy Table 54G/54.7

brman · Dec 4, 2012

spinlondon said:
The earthingnuts chart is based on the same information as John Peckham's chart.
The main difference being, that John has conducted the adiabatic equation for each of the cables, rather than just relied on Table 54G/54.7.
The earthingnuts chart does not refer to the ratio of conductivity between copper and steel.
All the earthingnuts chart indicates, is the CSA required to satisfy Table 54G/54.7

but the green/red section of the earthingnuts chart shows both the actual csa of the SWA and the copper equivalent unless I have completely misinterpreted it. It uses the K value from the tables (eg table 54.x) to ratio the csa of the conductor. So the black figures are actual swa csa and the brown figures give the copper equivalent to allow comparison to the required sizes.

I think the point is that the K value takes into account temperature co-efficient and heat capacity as well as resistance and it looks like it gives a smaller copper equivalent csa than your 8:1 resistance calc shows.
I don't know anything more than the earthingnuts chart gives but that is the way I read it.........

Deleted member 26818 · Dec 4, 2012

The K values are used to determine whether a conductor can withstand high current for a relativly short duration.
This is why when conducting the adiabatic equation, higher fault currents require CPCs with smaller CSAs.
Bonding conductors (and where PME conditions apply earthing conductors), are required to withstand low, medium and occasionaly high current for unspecified durations.
As such, the k values and the adiabatic equation are not applicable.
The conductor have to sized to withstand diverted neutral currents, not just from the installation where they are installed, but from other installations conected through extraneous-conductive-parts, service pipework and such.
Here's a link to a chart showing the conductivity of different metals: | Electrical conductivity | conductivity thermal |Comparisons of Materials: Coefficient of Thermal Expansion and conductivity of metal
This chart list the conductivity of annealed copper as 100, and steel as being between 3 and 15.
SWA is normally taken as being about 12.5, which is how the ratio 8:1 is determined.

Engineer54 · Dec 4, 2012

brman said:
The table I have used before is this one.... http://www.earthingnuts.co.uk/pdf/pvc_xlpe.pdf
It shows the copper equivalent CSAs although not the 8:1 you quote. Using this table it does show you can get away with the swa as bonding in some sizes, not really with PME though......

Throw that table chart away, it's more confusing than anything else!! Use the chart/table that John Peckham published as all the relevant adiabatic equations have already been conducted to each of the cables listed.

Neither that chart you have been using, or John Peckham's chart refers in anyway to bonding requirements, they are purely to be used as SWA, CPC compliance.... CPC and Bonding CSA requirements are not the same.

markc123 · Dec 5, 2012

If you're able to run a SWA to it, couldn't you just run a seperate earth in some flexible conduit alongside the SWA? yes it makes the job that bit more pricey, but it won't be anymore time consuming really.

malcolmsanford · Dec 5, 2012

markc123 said:
If you're able to run a SWA to it, couldn't you just run a seperate earth in some flexible conduit alongside the SWA? yes it makes the job that bit more pricey, but it won't be anymore time consuming really.

You can indeed Mark run a separate protective conductor outside of the SWA. The 2011 amendment highlighted this procedure for the first time in regulation 521.5.1 though it is a practice that as been going on in all my time in the industry.

brman · Dec 5, 2012

spinlondon said:
The K values are used to determine whether a conductor can withstand high current for a relativly short duration.
This is why when conducting the adiabatic equation, higher fault currents require CPCs with smaller CSAs.
Bonding conductors (and where PME conditions apply earthing conductors), are required to withstand low, medium and occasionaly high current for unspecified durations.
As such, the k values and the adiabatic equation are not applicable.
The conductor have to sized to withstand diverted neutral currents, not just from the installation where they are installed, but from other installations conected through extraneous-conductive-parts, service pipework and such.
Here's a link to a chart showing the conductivity of different metals: | Electrical conductivity | conductivity thermal |Comparisons of Materials: Coefficient of Thermal Expansion and conductivity of metal
This chart list the conductivity of annealed copper as 100, and steel as being between 3 and 15.
SWA is normally taken as being about 12.5, which is how the ratio 8:1 is determined.

Ah, thanks, I understand the differences now. Yet again I hadn't quite thought it through.

Engineer54 · Dec 7, 2012

Thought i'd post this table by John Peckham up again, as it seems folk are still getting confused with the other chart that's banded around on a regular basis. This table can be directly read as all the relevant adiabatic equations etc, have already been conducted.
Remember, this table is for checking the compliance of SWA cables for using the Armour as the CPC. They do not in anyway refer to bonding CSA compliance!!

BLUE = min CSA of SWA associated with corresponding cable size.
RED = (in brackets) Do Not comply

Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA	Minimum CSA of SWA to meet 54G	CSA of armour 2 core	CSA of armour 3 core	CSA of armour 4 core
1.5	3.4	15	16	17
2.5	5.7	17	19	20
4	9.0	21	23	35
6	13.6	24	36	40
10	22.6	41	44	49
16	36.1	46	50	72
25	36.1	60	66	76
35	36.1	66	74	84
50	56.4	74	84	122
70	79.0	84	119	138
95	107.2	122	138	160
120	135.3	(131)	150	220
150	169.2	(144)	211	240
185	208.6	(201)	230	265
240	270.6	(225)	(260)	299
300	338.3	(250)	(289)	(333)
400	403.9	(279)	(319)	467

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA	Minimum CSA of SWA to meet 54G	CSA of armour 2 core	CSA of armour 3 core	CSA of armour 4 core
1.5	3.4	16	17	18
2.5	5.7	17	19	20
4	9.0	19	21	23
6	13.6	22	23	36
10	22.6	26	39	43
16	36.1	41	44	49
25	36.1	42	62	70
35	36.1	62	70	80
50	56.4	68	78	90
70	79.0	80	90	131
95	107.2	113	128	147
120	135.3	(125)	141	206
150	169.2	(138	201	230
185	208.6	(191)	220	255
240	270.6	(215)	(250)	289
300	338.3	(235)	(269)	(319)
400	451.0	(265)	(304)	452

ziggless · Dec 7, 2012

spinlondon said:
Yes 10mm² bonding or TT the shed.

i agree

Darkwood · Dec 7, 2012

Cant you just tell the customer wooden sheds are the in thing now

SirKit Breaker · Dec 7, 2012

whatever you do, shove an earth rod in anyway.

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

Engineer54 · Dec 7, 2012

ziggless said:
i agree

There would have to be a very good reason, to TT a shed that was only 5 metres away from the main house supply!!

brman · Dec 7, 2012

Engineer54 said:
Thought i'd post this table by John Peckham up again, as it seems folk are still getting confused with the other chart that's banded around on a regular basis.

Good move! I picked up the other chart from here somewhere and it obviously confused me