Maximum Ze value for TNS and TNCS System

[dave1064]

I am wondering where the maximum values (0.8 and 0.35 ohms) come from. Why are they 0.8 and 0.35?

Any help would be greatly appreciated

 

[telectrix]

they come from BS7671. same as a lot of other useful guff.

 

[NMc]

Thought it was the ESQCR

 

[telectrix]

that as well.

 

[pistolpete]

Tn-s =0.28ohms??

 

[Engineer54]

Tn-s =0.28ohms??

Does it??

Or are you asking or stating?

 

[telectrix]

it may well be. 0.8Ω is the max. allowed value.

 

[Fox]

I am wondering where the maximum values (0.8 and 0.35 ohms) come from. Why are they 0.8 and 0.35?

Any help would be greatly appreciated

Have a think about how the distributors means of earth is supplied on each supply characteristic. Why do you think you may record a higher impedance on a TNS supply?

 

[dave1064]

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?

 

[telectrix]

because someone in a suit with letters after his name formed a committee and those values were the average taken out of the hat.

 

[Fox]

A TNS (Terra Neutral Seperate) system will utilise the metallic sheath on the supply cable as a means of earth. During the event of a fault, the fault current will flow down the outside sheath of the supply cable through the ground, back to the suppliers source of earth.

A TNC-S (Terra Neutral Combined System) uses the neutral conductor as a means of earth. Again, in the event of a fault, the current will flow back to the supply transformer through the neutral conductor.

Overall, a TNCS system should and will boast a lower overall fault loop impedance as the neutal conductor is of far lower resistance (impedance in an AC circuit) than a lead sheath running underground back to the supply transformer.

Are you at college? Have you asked your tutor?

 

[dave1064]

Im a 4th year apprentice due to sit my trade test in 2 weeks. So been studying etc. And the question was asked why it is 0.8. Just wondered how they came up with 0.8 ohms

 

[Fox]

Have a look at tel's reply - it was probably calculated as an average using lead resistivity values etc - you won't specifically need to know why it's 0.8 Ohms.

So long as you know that 0.8 Ohms is the maximum value allowed on a TNS system, and a Ze reading will generally be far lower. Try to understand the principle of Ohm's Law and why different earth types have different maximum values.

Good luck

 

[telectrix]

it's also a fact that while TN-S sheath is earthed at the sunstation ( apart from contact with the ground) a TNC-S system has multiple earthing points along the length of the supply cable (protective multiple earthing, PME) and that will reduce the Ze as well.

 

[Fox]

is earthed at the sunstation

That's our local tanning centre!

 

[telectrix]

haha, why are the keys smaller than my fingers. quite often get the next one to the one i want. same thing happened when i got married. got the ex instead of her younger sister .

 

[Fox]

I made that same mistake!

 

[telectrix]

so you got the rottweiler instead of the vixen.

 

[Engineer54]

it's also a fact that while TN-S sheath is earthed at the sunstation ( apart from contact with the ground) a TNC-S system has multiple earthing points along the length of the supply cable (protective multiple earthing, PME) and that will reduce the Ze as well.

In theory, the lead sheath of PILC cables should never come in direct contact with the ground there was a insulating/bedding material between the old network wave line armour and the lead sheath. It is generally the breakdown of that insulating material allowing corrosive soil types to degrade/corrode the lead sheathing. Best ask Tony what the make-up was of those older PILC Waveline cables were, he has far more experience with them than i!!

Like the PME distribution cables, the TN-S cables would gain supplementary earthing from installations via bonding cables and connections to steel frame buildings and the like. One reason why they used to rod at TN-S service heads during the 50's. But as you rightly say PME cables will also (in theory anyway) have it's PEN conductor physically connected to earth at several points along it's length.

 

[dave1064]

Cheers guys. I know it wont come up but was just curiousity. That was the only explanation I could give. Just thought there might have been more to it.

 

[Outspoken]

earthed at the sunstation

But Tel, surely that is only for Solar Installations

 

[telectrix]

be a bloody long and expensive cable to earth the sun. would need to have a high operating temperature.

 

[Outspoken]

be a bloody long and expensive cable to earth the sun. would need to have a high operating temperature.

I think you would be struggling even with Pyro!!

 

[trev]

be a bloody long and expensive cable to earth the sun. would need to have a high operating temperature.

You could have it disconnected during the day time.

 

[Outspoken]

You could have it disconnected during the day time.

So that would mean the Australians would not have an Earth-Sun link during their day...Oh that would grate, they would moan like old dears!!

 

[telectrix]

You could have it disconnected during the day time.

sheer class.

 

[Darkwood]

As far as I know the values we are given for max or requirements for the DNO to comply to and if you are designing an install that doesn't yet exist you need a set max figure to base your design calculations on, the difference is as already mentioned as its expected a TNCS can achieve a better reading than a TNS as it has multiple earth points along its length.

 

[PEG]

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:

 

[Darkwood]

My question is; Is the sun Inductive or resistive :ciappa:

 

[Outspoken]

My question is; Is the sun Inductive or resistive :ciappa:

Convective!

 

[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