what is best tn-s or tn cs

[consumer done over]

probably this question has been asked before but in a nutshell which of the two system is better?

I have an eartblock connected to the incoming supply via a small green wire. The earthblock is connected to gas, water main and also goes to the main CU.

Ze on test certificate I was given reads 0.12 not that this means anything to me.

 

[RF Lighting]

They both have their own merits.

It depends on the installation type. For example, TN-C-S can be a problem on farms, and is not permitted on caravan sites.

TN-S often does not provide as low earth loop readings which can be a problem for some installations.

 

[celticrsuperb]

tn-s systems r better for house hold circuits because you have a sperate earth and neutral, instead of having havin your earth coming of your neutral. its alot easier to understand

 

[RF Lighting]

But it's more often than not higher resistance supply.

Why would being easier to understand (in your opinion) make it a better system?

I'd choose a TN-C-S every time for a standard domestic supply, and I suspect so would most other folk on here too. (and all the DNOs)

 

[ban-all-sheds]

its alot easier to understand

WTF

Don't be ridiculous.

 

[consumer done over]

They both have their own merits.

It depends on the installation type. For example, TN-C-S can be a problem on farms, and is not permitted on caravan sites.

TN-S often does not provide as low earth loop readings which can be a problem for some installations.

I see.
Not being a pro I'm wondering how many of those little green wires that connect the earth block to the supply are disconnected without the owners knowing. Does this happen a lot?

Also I'd like to know if the Ze reading can be ok even with that wire disconnected or would a bad reading signify the existence of such issues (or indeed a problem with the incoming supply earthing provided).

 

[plugwash]

TN-S is a safer system IMO, with a TN-C-S all it takes is one bad joint in the neutral in the wrong place to make everything live. There can also be significant currents running down your main bonding which could potentially be a problem.

Whether the extra safety is enough to make up for the extra cost of TN-S is doubtfull though. Also there are stricter rules on allowed Ze with TN-C-S which can be very usefull for some installations.

 

[Click-Sure]

I see.
Not being a pro I'm wondering how many of those little green wires that connect the earth block to the supply are disconnected without the owners knowing. Does this happen a lot?

Also I'd like to know if the Ze reading can be ok even with that wire disconnected or would a bad reading signify the existence of such issues (or indeed a problem with the incoming supply earthing provided).

Connected without the owners knowing? well, not exactly, they would have been connected when the supply was first installed and most have not changed since!

 

[ban-all-sheds]

Also there are stricter rules on allowed Ze with TN-C-S which can be very usefull for some installations.

I know that Ze is assumed to be lower - didn't know it had to be...

 

[plugwash]

I thought there was a requirement in the ESQCR but having just taken a look if it is there I can't seem to find it.

 

[sparkybird]

The other problem I have found with TNCS/PME is that the PSCC can be very high, verging on the 6kA which is the limit of most type B mcbs...

 

[RF Lighting]

Also there are stricter rules on allowed Ze with TN-C-S which can be very usefull for some installations.

I know that Ze is assumed to be lower - didn't know it had to be...

Max permitted Ze values:

TN-S : 0.8 ohms
TN-C-S : 0.35 ohms

 

[ban-all-sheds]

Those are the maxima for installations to which the OSG is applicable, because they are assumed in the tables for circuit lengths etc.

But where are they defined as the maximum permitted values? They are what they are, and will affect the maximum R1+R2 values permitted so that the maximum permitted Zs values are not exceeded, but unless there is a definitive requirement in the ESQCR which forbids suppliers from allowing their Ze to exceed those limits then they aren't maximum permitted values...

 

[Spark123]

iirc they are the maximum figures specified by the distributor.

 

[NotHimAgain]

The values of Ze (TN-C-S = 0.35; TN-S = 0.8 and TT = 21 ohms) originally came from the Electricity Council (now defunct and replaced by the Electricity Association). These are / were the supply industry joint technical bodies.

The figures do not appear in the law but they represent good supply industry practice so they are usually used.

Suppliers are required by law to tell you the maximum prospective short circuit current at the supply terminals and the maximum Ze (ESQCR 28(a) & (b)). If asked they will often quote 16KA (at the service to main joint in most cases, but at the supply terminals in some cases) and a Ze of 0.35 ohm for a 100A single phase TN-C-S supply. These values are actually mutually exclusive.

Now the actual regulation calls for these values to be "which apply, or will apply, to that installation" but you try getting them to give you tailored figures