MAXIMUM Ze at origins of TT, TN-S & TNC-S,

[mgnelec09]

Hello there,

I know this may sound like a daft question but I am currently looking at A job where the Ze reading is 0.37 in a distribution board.It looks to be a TNC-S system by checking where the board is fedd from at the mains cut- out, the run from the mains is about 70 to this board and they are wanting to add 3x 63 amp circuits.

Does this reding seem to be Ok? The amount of people who have given me different answers over the phone.

Could someone please clarify Max Ze readings for all 3 systems?


Many Thanks

 

[Liamsparks]

Hello there,

I know this may sound like a daft question but I am currently looking at A job where the Ze reading is 0.37 in a distribution board.It looks to be a TNC-S system by checking where the board is fedd from at the mains cut- out, the run from the mains is about 70 to this board and they are wanting to add 3x 63 amp circuits.

Does this reding seem to be Ok? The amount of people who have given me different answers over the phone.

Could someone please clarify Max Ze readings for all 3 systems?


Many Thanks

TN-CS (PME) 0.35ohms
TNS - 0.8ohms
TT - Dependent on means of earthing and csa normally arround 1667ohms but anything over 200ohms should be investigated. In english its reccomend that its under 200ohms i.e put another stake in thats up to you.

 

[Electricalserv]

Max Ze for a tnc-s is 0.35ohms, but could be your meter out of scale etc.

 

[jumpin jax]

it sounds as though you are measuring Ze at a distribution board and not at the origin, also it is allowed for the Ze to be greater than 0.35 ohms but the cut out fuse must be reduced from 100A accordingly

 

[telectrix]

yeah, where are you measuring your Ze. it's the Zeat source that's max. 0.35.if you are measuring at a DB 70m? from source, you will have the added res. of the feed cable and that Ze/ should be recordede as Zdb.

 

[scotsparky]

TN-CS (PME) 0.35ohms
TNS - 0.8ohms
TT - Dependent on means of earthing and csa normally arround 1667ohms but anything over 200ohms should be investigated. In english its reccomend that its under 200ohms i.e put another stake in thats up to you.

Remeber these readings are the maxuimum of the supply authoritys side.
TT from the DNO is 21 ohms which is the resistance of there rod at the transformer. A max of 200 it for the Installation side in a TT system so be carefull.

 

[mgnelec09]

Sorry yes the measurement taken of 0.37Ohms was at the distribution board about 70 metres away from the cut out.

The reading at the cut out/ main incomer was around 0.15 Ohms I think.

Does this mean my 0.37 is ok? as they are asking for another board to be installed aboiut 15 metres away powered from this one?

 

[scotsparky]

0.37 may be fine depending on the overcurrent device being used.

your ZeD.B is 0.37ohms what is your ZeD.B for the new board going to be? you will need to calculate that as well.Then work out the ZS for the final circuits. whit the Zs values you can select an overcurrent device to use.

remember your correction factors,Volt drop, ect

 

[Deleted member 26818]

There is no maximum allowed Ze, the values are just guidlines.
Now from the information you have supplied, your Ze is 0.15Ω, and apparently your Zdb is 0.37Ω, is this measurement with or without bonding conductors connected?
Zdb should be measured with the bonding conductors connected.
As for the reading being OK, we would not be able to answer that, as we don't know what has been specified by the designer, nor what the R1 + R2 of any final circuits are or their protective devices.

 

[mgnelec09]

This reading is with the bonding conductors in place, nothing has realy been specified as it is a private company who seem a bit gung ho!hence me being a little paranoid about there proposed ideas.

Thanks for your input, much appreciated.
They want everything signed off and certified, which I am qualified to do. But for piece of mind, I just needed a little re assurance.

 

[Monkeyblaine]

Why does the cut out fuse need to be reduced for greater ze readings. Not being funny just having a dumb day. Thanks.

 

[Silly Sausage]

Why does the cut out fuse need to be reduced for greater ze readings. Not being funny just having a dumb day. Thanks.

The fault current (230/Ze) could be too low to ever take the fuse out.
Look at the Time/Current graphs in your regs book.

 

[Monkeyblaine]

Thank you.