100A 3-Phase supply header

[stuartsjg]

Hi,
Can somebody please post a pic showing a 100A 3-phase supply header with the covers and fuses removed but the wires in place?

Im an electrical engineer and am having a non-meeting of minds with a senior colleague over our office supply.

He thinks the earth is the cable armour and there are 4 cores (3p+n). The neutral connects to the star point of the sub station and the earth goes via a NGR.

Im convinced that is not and its a 3 core cable and the armour forms the earth and neutral which is solidly connected to the Tx star point.

Unfortunately i cant remove the terminal plate which will show the earth and neutral being screwed together (if im right which im sure i am) without taking out the phase fuses which may upset those trying to work.

So if somebody has a good pic showing the insides of a header that would be great!

Thanks,
Stuart

 

[nozspark]

I don't have a picture, but both of you are right...

If it's a 3 phase TNS supply via SWA then you will have 4 cores (L1, L2, L3 & N) and the armour sheath as earth

however,,, if it's a 3 phase TNC-S supply via SWA then you will have 3 cores (L1, L2 & L3) and the armour sheath will be your earth and neutral

 

[Lectrician]

It will be fed via a 'waveform concentric' cable. Not SWA.

It maybe a three or four core waveform. The outer concentric cores of the cable will be copper, not steel, and are concentric clockwise, then anticlockwise, then clockwise, then anticlockwise (hense waveform).

If it is a TN-S supply, It will be a 4 core waveform, the outer concentric being just the earth.

If it is a TN-C-S supply, it will be a 3 core waveform, the outer being a CNE / PEN conductor (concentric neutral earth / protective earthed neutral).

Not sure what you mean by NGR?

All supply types end up with the neutral and earth returning to the substations star point by some means or another.

 

[Lectrician]

Here is a TP head typically used in industrial units in my DNO's area.

This is a TN-C-S / PME supply. Notice the 3 core waveform.

I can't say it is the neatest termination!

Untitled

• Lectrician
• 29 May 2009
• 1

 

[ban-all-sheds]

Unfortunately i cant remove the terminal plate which will show the earth and neutral being screwed together (if im right which im sure i am) without taking out the phase fuses which may upset those trying to work.

Why not look at the last PIR to see what it says about type of supply?

 

[stuartsjg]

Thanks for the info, im not sure if its TN-C-S or TN-S.

The header is one of the Lucy Switchgear TPNE100 units and the neutral and earth blocks look to be under the same plate and there are 3 fuses., if it was a TN-S then id have thought there would be clearly separate neutral and earth terminals.

By NGR i mean neutral grounding resistor.

We commonly work with offshore electrical as well as land power station systems where the earth has a resistor connecting it to the neutral star point.

This ensures that the line-earth fault current is not the full short-circuit current and generally limits the current from many 10's of kA to perhaps no more than 1-2000A, enough to trip what ever the protection systems are quickly, but not so much as to blow up the fault point beyond recognition.

Generally small transformers wont have them and if its a TN-C-S there defiantly wont be one.

The other guy has always been used to working with 10-2000MVA and upwards systems where you will never have the neutral and earth hard connected - i think hes forgotten about the smaller end of the scale!!

Some pix on http://www.eon-uk.com/distribution/901.aspx in the underground cable section shows a single phase underground connection with the CNE coming from the copper braiding.

Ill see if i can see any markings on the cable showing the core count.

Thanks,
Stuart