3 Phase Install

[Danny258]

Hi All,
Happy new year to you all!

I’m working for an engineering company, been here for a few years.

im currently working towards my NVQ level 3 in electrical maintenance and I’m in the middle of supplying a 100A Distribution board from a newly upgraded main 250A board. When the new board was installed I asked the sub contractors to run a armoured cable from this new board to the 100A board but they only supplied a 4 core 25mm cable.

Obviously this 100A board needs 3 phase, neutral and earth (5 core)

I have read somewhere that the armour of the cable can be used as the earth?

personally that doesn’t sound good practice?

any advice will be much appreciated! Thanks

 

[sinewove]

Hi All,
Happy new year to you all!

I’m working for an engineering company, been here for a few years.

im currently working towards my NVQ level 3 in electrical maintenance and I’m in the middle of supplying a 100A Distribution board from a newly upgraded main 250A board. When the new board was installed I asked the sub contractors to run a armoured cable from this new board to the 100A board but they only supplied a 4 core 25mm cable.

Obviously this 100A board needs 3 phase, neutral and earth (5 core)

I have read somewhere that the armour of the cable can be used as the earth?

personally that doesn’t sound good practice?

any advice will be much appreciated! Thanks

Standard practice to use the armour as cpc,but not normally used as a main bonding conductor.
regards,S

 

[Danny258]

I have now used the armour as the earth but I have also run a separate main 25mm earth. Is this ok? In regards to wiring regulations?

 

[loz2754]

personally that doesn’t sound good practice?

What would be good practice would be to have the new board and its supply designed properly by a qualified and competent person in accord with EAWR.
Who will be signing the Electrical Installation Certificate as the designer?

 

[Danny258]

What would be good practice would be to have the new board and its supply designed properly by a qualified and competent person in accord with EAWR.
Who will be signing the Electrical Installation Certificate as the designer?

Once the install is complete we will have a approved contractor to sign the work off. As I'm working towards my NVQ I need to create job reports. Would like to get everything right before sign off

 

[Pete999]

Hi All,
Happy new year to you all!

I’m working for an engineering company, been here for a few years.

im currently working towards my NVQ level 3 in electrical maintenance and I’m in the middle of supplying a 100A Distribution board from a newly upgraded main 250A board. When the new board was installed I asked the sub contractors to run a armoured cable from this new board to the 100A board but they only supplied a 4 core 25mm cable.

Obviously this 100A board needs 3 phase, neutral and earth (5 core)

I have read somewhere that the armour of the cable can be used as the earth?

personally that doesn’t sound good practice?

any advice will be much appreciated! Thanks

The Steele wire may be suitable as a earth for the new install, however if the new supply requires any bonding, due to any metallic items in the building, such as water ,gas A.C or oil pipework, entering the building from the outside,then you will probably find that the armouring won't have the required CSA to act as a main earth AND main bonding conductor, something the designer/contractor that initially did the work should have taken all this into consideration at the outset, a poor design in my book.

 

[loz2754]

a poor design in my book.

Or NO design.

 

[Danny258]

The Steele wire may be suitable as a earth for the new install, however if the new supply requires any bonding, due to any metallic items in the building, such as water ,gas A.C or oil pipework, entering the building from the outside,then you will probably find that the armouring won't have the required CSA to act as a main earth AND main bonding conductor, something the designer/contractor that initially did the work should have taken all this into consideration at the outset, a poor design in my book.

great, thank you for the advice

 

[Strima]

What did your design calculations show for volt drop, expected PFC, Zs, adiabatic etc?

 

[DPG]

Is someone overseeing your work? Who is doing the calculations?

 

[westward10]

The chances of the armour being inadequate as a cpc is pretty slim providing the cable is correctly terminated and correct circuit protection is provided. You should however prove this. The use of cable armour and steel containment as a cpc seems to have become a no no when in fact for most applications it is perfectly acceptable.

 

[pc1966]

As above, in most cases the armour is perfectly adequate from an adiabatic limit point of view, but should be checked anyway as some cases it is not.

However, it might not be enough for TN-S-C bonding, and it might not have a low enough impedance to meet the end of cable Zs for disconnection times.

25mm 4-core has about 70mm of steel, which is about 8.75mm copper equivalent and too low for TN-C-S bonding that demands 10mm. Also the R2 is about 2.3 mOhm/m, compared to the R1 of about 0.8mOhm/m so on a longer run it dominates your Zs term.

If you have run a 25mm copper line in parallel the problems go away, but that ought to be driven by design, not as a seat-of-pants reaction.

 

[Flanders]

If it's not been designed and just guest. Then you need to check the voltage drop on the cable . You will need various information to work this out including, max demand ,method of installation and length of run