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.