Maximum Ze value for TNS and TNCS System

[Outspoken]

The power of the sun is known,and it's 93,000,000 miles away,so what size copper cable to get disconnection in 5 secs?...and if it has an incomer,is the Lord DNO? Who's gonner cut THOSE seals? :yes:

The Sun breaches BS7671 because even if the electrons traveled at the speed of light, which they do not, the disconnection time still exceeds that allowed. The minimum is 8m 7s and the maximum would be 10m 1s depending on where the Earth is in it's orbit. Not figured out yet how to word the deviation on the test cert!!

However there is a rumour the Vogons want to use the cable to power street lighting on their interplanetary highway, the cable will be surplus once they destroy the Earth to make way for said highway..according to Arthur Dent anyway!!

 

[Baker1988]

I know where they come from. I am just wondering why the maximum is 0.8 for a tns system. Why not 1ohm or anything else. What is the reason?

Hi correct me if i am wrong but i think you are all thinking way too much into this it is only a college question not a hnd question.

My answer would be they are set at a maximum 0.8 ohm impedance for a tns system so that if a fault was to occur, due to the low impedance you would get a high fault current which would operate the over current/load device quicker which could save the life off a human or cattle.

That is essentially why it is low, and as u said "why is it not 1 ohm" well cos a 1 ohm impdence would create a 230amp fault current and that would not trip some of the higher rating and higher breaking capacity devices in the required time so some one could be killed.

And this is where the rcd debate rears its ugly head again

 

[Outspoken]

I'll have to have a hunt in my old books, but I seem to recall that some of the values, not saying these specific ones, date to the early days and are related to the lightning protection systems on larger building, the value of the supply cable return path having to be significantly less than the lighting protection of a building to avoid the protection becoming live in the event of a fault.

I could be way off base here, it's a long time since I have given this any serious thought, just use the values supplied..but there is some memory in my old head rattling about on this, been driving me mad since I first read this accursed thread!!

Update

Aha, quick thought process into some pdf's I have.. IEC 62305 states that all Air and ground termination systems for Lightning protection should have an impedance of no more than 10Ω, and recommend it should be significantly lower is possible with electrode depths from 3-9m depending on soil type. So we are seeing that any supply cable should always aim to be significantly lower than this because fault currents need to return to ground to dissipate and not dissipate in the lightning protection system as the CPD may no operate.

Will keep hunting...

 

[Engineer54]

Think you'll find originally on TN systems, it had far more to do with limiting the time a fault can be sustained before protective devices operate. In this instance the service cut out fuses.

On TNC-S systems it isn't always a max of 0.35ohm!! That figure relates only to full PME systems, it can and often is, significantly higher on partly converted systems that are not yet up to full PME standards.

 

[Outspoken]

You could be right there E54