No Earth from Supply on front of terraced house

[securespark]

Yes BS 7671-2008 (17th edition ) chapter 54 Table 54.1.Besides which the rule of thumb is Half the size of the main Incomming phase conductor.

You sure that's the rule on TT-fed supplies? Sure 6mm² won't suffice?

Neil. not sure from your posts if you have RCD protection. At least add an RCD on the tails for now. It won't comply, but it will give you temporary protection against the high EFLI.

If you have TT, you MUST have RCD protection.

 

[scotsparky]

Its not supply specific. It is the rules. I have a susspition that you have got 6mm as a size from the suplimentary bonding sizes which is different and 6mm is fine. However as the cable to the earth rod will have to carry any earth fault from any equipment or at worse care the fault of a dead short at the D.b itself it has to be bigger
Its easy to get the earth termanoligy confused.
Main earth conductor ------ is supply Min 16mm
Main Bonding Conductor ----- to water,Gas,structural steel 10mm in houses can change in commertial industial installations.
Supplimentary bonding---- between pipes sinks ect 4mm
Circuit Protective conductor---- it the seperate earth for each circuit. sizes vary with each circuit.
Sizes above are minimum to be used.

 

[aptsys]

Question: What might be a best case likely current to earth on a TT supply caused by a dead short phase to earth?

 

[securespark]

Supplimentary bonding---- between pipes sinks ect 4mm

In every case?

Scotspark, in both cases (supplementary [generally] & earthing conductors [for TT supplies]), the csa can be as low as 2.5mm².

See tables 4.2 and 4.3 in the OSG.

Question: What might be a best case likely current to earth on a TT supply caused by a dead short phase to earth?

Let's say best case scenario for Ze: 22 ohms. Highest voltage 252.

252/22 = 11.45A

 

[scouseneil]

Yes it has an RCD thanks. Interesting that several neighbours don't and some appear not to have earth rods.

Again many thanks to everyone.

 

[aptsys]

Question: What might be a best case likely current to earth on a TT supply caused by a dead short phase to earth?

Let's say best case scenario for Ze: 22 ohms. Highest voltage 252.

252/22 = 11.45A

Exactly

16mm² would make very little sense except in some remote cases.

 

[ColJack]

I take it that this is a hypothetical TT system with no RCD fitted?

so at 11.45A, your main fuse wouldn't blow.. so your pipes and everything are then at 252 volts compared to real earth...

you grab it and become part of the circuit to earth..

for arguments sake lets assume the following..
1. the human body has a resistance of 500ohms when dry..
2. you are barefoot.. and touching a metal pipe
3. the resistance of the metal pipe is negligable..

so that's 1/Rt = 1/R1 + 1/R2 ( etc.. )

1/Rt = 1/22+1/500 = 21.07 ohms

which is 11.96A

of which 11.45 will be through the 22 ohm earth wire, and the remaining 510mA through you...

 

[plugwash]

16mm² would make very little sense except in some remote cases.

IIRC it is required if the earth runs underground with protection against corrosion (plastic insulation) but without protection against mechanical damage.

For running bare earth conductors underground (which can make a very effective earth electrode) IIRC there is a 25mm minimum

But yeah if it's not running underground 16mm is overkill for a TT main earth.

 

[securespark]

If you're running an earthing conductor underground for TT and it is protected against corrosion & mechanical damage, then you can use 2.5mm².

 

[scotsparky]

Secure spark if you look at 7671-2008 page 130 table 54.7 it gives you minnimum sices of protective conductor. Table 54.1 0m page 127 is for TN-C-S where pme conditions apply systems as stated in 542.3.1 If its pme then there is a huge amount of parrallel earths includint the metalwork of the building. This cannot be relied on in tt systems.
I personnaly would code1 any installation that i found that had a 2.5mm as a main bonding conductor.

 

[scotsparky]

as for the the size of the suplimentary bonding read page 135 of BS7671-2008 reg 544.2.2

 

[securespark]

Scotspark, in both cases (supplementary [generally] & earthing conductors [for TT supplies]), the csa can be as low as 2.5mm².

See tables 4.2 and 4.3 in the OSG.

You can quote your tables in the regs.

However, have you seen the tables I refer to?

From table 4.3, supplementary bonding conductors between extraneous conductive parts where mechanically protected can be 2.5mm².

Similarly, table 4.2 lists the csa of the earthing conductor (either buried or non buried, protected against corrosion & mechanical damage) as 2.5mm².

How can you justify giving a code 1 to an installation that complies fully with BS 7671 2008?

 

[Spark123]

Secure spark if you look at 7671-2008 page 130 table 54.7 it gives you minnimum sices of protective conductor.

It maybe does, however if you read 543.1.1 you may also use 543.1.3 to calculate the size using the adiabatic equation. This doesn't get you away from the other requirements for minimum sizing tho.

Table 54.1 0m page 127 is for TN-C-S where pme conditions apply systems as stated in 542.3.1 If its pme then there is a huge amount of parrallel earths includint the metalwork of the building. This cannot be relied on in tt systems.

No it isn't, it is the table for the minimum size for buried earthing conductors.
542.3.1 says for all types of supply the earthing conductor must comply with the minimum sizes by section 543 and where the supply is PME you it must also comply with minimum sizing requirements of regulation 544.1.1 for the sizing of PEBs

I personnaly would code1 any installation that i found that had a 2.5mm as a main bonding conductor.

Seems reasonable, given reg 544.1.1 calls for a minimum PEB of 6mm²

 

[scotsparky]

Sparks 123 54.1 is for burried cables bit 542.3.1 says "Every earthing conductor must comply to section 543 and where pme conditions apply and in addition where burried in ground so you only use 54.1 afet making sure you comply with section 543.
Yes you can calculate using the adiobatic 543.1.3 or if you dont do that table 54.7 can be used see reg 543.1.4.

 

[ColJack]

ok, so from my understanding... ( and rather simplified )

the MAIN EARTH has to be calculated via the adiabatic, but as a general rule of thumb must be...

same size as live if the live CSA is less than 16mm²

16mm² if live CSA is between 16mm² and 35mm²

half the size of the live for a live CSA bigger than 35mm²

============================

PEB's

MPB ( not the same as main earth.. these are bonds to water, gas, structural steel etc )

non PME supplies must be between 6mm² and 25mm² ( providing it's copper ) and not less than half the size of the main earth.. so a not less than a quarter of the size of the live.. ( given a typical supply with 25mm² tails, this works out to 6.25mm² minimum, so 10mm² is needed )

PME supplies, must comply with table 54.8 ( ie 10mm² for typical 25mm² tails.. )

suplimentary bonds minimum 2.5mm² if mechanically protected, 4mm² if not..

now the minimum sizes stated in table 54.1 are for buried conductors.. so as long as they comlpy with the above, you can have a 2.5mm² copper, mechanically protected conductor burried ( for example to an earth rod )
this does however mean that the supply tails would also have to be 2.5mm²...

the MAIN EARTH is not a protective bonding conductor, it is a circuit conductor and must conform to the rules set out for such ( as listed above )