Zs to high on MCCB Protected circuit?

[davesparks]

Hi I haven't taken the Ze or Zs yet. This cable has recently been installed and has not been energized. The cable is a 4c 70mm SWA feeding a 3ph distribution board 550 mtrs away buried in the grounds. The source is a 160amp 3ph Panel board in a stately home. The r1+r2 is 0.8Ohm so despite any other impedance readings the Zs of this supply shall not be low enough to comply with Schneider max Zs of 0.38 for this device. Due to the size of the cable and the lack of space in the panel add-ons are not an option.

Are there any adjustable settings on the MCCB high need to be set according to the design to achieve compliance?

Alternatively the designer is looking at one hell of a claim!

 

[biospark80]

The lowest setting is 35amps but on the Schneider design Max Zs is still 0.38. Trying to find a mccb with earth leakage trip.

 

[Simlec]

You just need to send the MCCB back to Schneider (only they can install the trip units) and have a different tripping unit fitted (micrologic 2.2 if my memory serves me correct) with more adjustability for setting your tripping curve.
You don't want to be putting and earth leakage unit on it, it's a distribution circuit.

 

[Andy78]

The lowest setting is 35amps but on the Schneider design Max Zs is still 0.38. Trying to find a mccb with earth leakage trip.

Why are you trying to find solutions to this problem which obviously lies with the designer and installer of this cable ? what's your extent of involvement here ?
You say in your OP you are checking a cable, are you doing the initial verification ?
What horrible figure have you arrived at for volt drop ?

 

[polo1]

Thought I'd bump up this post ' cos I'm really interested in the outcome! OP, you there???

 

[biospark80]

Hi Iv'e spoken to Schnieder technical and they have come up with a a vigirex unit and toriods which are essentially a rcd unit with ct's. This shall give the additional protection required due to the high Zs.

 

[Flanders]

Why are you trying to find solutions to this problem which obviously lies with the designer and installer of this cable ? what's your extent of involvement here ?
You say in your OP you are checking a cable, are you doing the initial verification ?
What horrible figure have you arrived at for volt drop ?

The op as not mentioned that but I have done a quick basic calc and I got a volt drop of around 30 volts at 100 amps, 550 meters seems way to long for this size of supply I am also interested to find out how the correct volt drop was achieved, You would need to go down to about 25 amps max load to get any were near

 

[Andy78]

The op as not mentioned that but I have done a quick basic calc and I got a volt drop of around 30 volts at 100 amps, 550 meters seems way to long for this size of supply I am also interested to find out how the correct volt drop was achieved, You would need to go down to about 25 amps max load to get any were near

I don't think it has been. It doesn't sound as if the calcs have been done correctly for this install which is why I brought the volt drop up, as the OP seemed only concerned about the high EFLI value and maybe hadn't considered VD in his solution.

 

[Flanders]

I don't think it has been. It doesn't sound as if the calcs have been done correctly for this install which is why I brought the volt drop up, as the OP seemed only concerned about the high EFLI value and maybe hadn't considered VD in his solution.

Yes I agree For 160 amp I think you would would be looking at 2x 300mm in parallel ( I'm only Guessing By the way) I think sorting the Zs is the least of his worries , I don't think any calcs have been done I thinks someone looks in regs on the table for cable size for swa seen that a 70mm will take 173 amp underground and said that will do it !!!

 

[Andy78]

Yes I agree For 160 amp I think you would would be looking at 2x 300mm in parallel ( I'm only Guessing By the way) I think sorting the Zs is the least of his worries , I don't think any calcs have been done I thinks someone looks in regs on the table for cable size for swa seen that a 70mm will take 173 amp underground and said that will do it !!!

What's worrying me is that the stated load at the end of this distribution circuit is a TP+N DB. We don't have any indication of the loading of the DB but it's not going to be insignificant.
There is a lot of missing info about this whole situation, but the designer of this circuit needs bringing into this situation asap.

 

[Flanders]

It may be OK if its just a one light and a socket but I don't think that is going to be the case we will have to see if the OP comes back with any more info :willy_nilly:

 

[biospark80]

It may be OK if its just a one light and a socket but I don't think that is going to be the case we will have to see if the OP comes back with any more info :willy_nilly:

Hi Guys some people are clearly not readings this thread from the start. To clear some details up,
Source - 160 3ph+n panel board.
MCCB- 50/35a settings max Zs 0.38 Ohms
Cable - 550mtrs of 4c 70mm Xlpe swa
Load - 3ph-n Db but only requires 35amps of load to be used.

The r1 + r2 value of the cable is 0.8 Ohms measured, which clearly shows that the Low Max Zs is not obtainable.
Schnieder has now come back to me and recommended a Vigirex unit with toroids. This kit will give me an adjustable range of protection from 0.3-1 amp earth leakage current on the cable which shall bring it inline with BS7671.

 

[Flanders]

Hi Guys some people are clearly not readings this thread from the start. To clear some details up,
Source - 160 3ph+n panel board.
MCCB- 50/35a settings max Zs 0.38 Ohms
Cable - 550mtrs of 4c 70mm Xlpe swa
Load - 3ph-n Db but only requires 35amps of load to be used.

The r1 + r2 value of the cable is 0.8 Ohms measured, which clearly shows that the Low Max Zs is not obtainable.
Schnieder has now come back to me and recommended a Vigirex unit with toroids. This kit will give me an adjustable range of protection from 0.3-1 amp earth leakage current on the cable which shall bring it inline with BS7671.

Ok thanks for coming back to us and sorry for the miss understanding ,what was the calculated volt drop on it cable ? must of been close to the max

 

[shanky887614]

if the supply is 0.8 then surely the circuits coming off it wont comply

 

[Leesparkykent]

if the supply is 0.8 then surely the circuits coming off it wont comply

Why is that?