Zs to high on MCCB Protected circuit?

[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?

 

[shanky887614]

Why is that?

0.8ze for a distribution circuit is quite high with other circuits coming off it, maybe not now but in the future when they go to expand.

just did a quick google search and 4c 70mm swa is £23m.

for 500m thats 12.5k surely it might have been a better design for another supply from the grid.

especially if they only needed a 30amp supply

 

[Leesparkykent]

0.8ze for a distribution circuit is quite high with other circuits coming off it, maybe not now but in the future when they go to expand.

just did a quick google search and 4c 70mm swa is £23m.

for 500m thats 11k surely it might have been a better design for another supply from the grid

Or two cables in parallel maybe. Back in the day when I was subbing we once pulled in a cable cant remember the distance but I know the cable cost 12k and it come up about 50meters shy of where it was supposed to go lol. Btw the measuring/ordering of the cable had nothing to do with me lol.

 

[Flanders]

0.8ze for a distribution circuit is quite high with other circuits coming off it, maybe not now but in the future when they go to expand.

just did a quick google search and 4c 70mm swa is £23m.

for 500m thats 12.5k surely it might have been a better design for another supply from the grid.

especially if they only needed a 30amp supply

but the OP said he was putting in an upfront rcd would that not cover the disconnection time for all final circuits aswell , saying that i don't think the design is the best but we don't know the resons for the design and there could well be a good reason fot it

 

[shanky887614]

but the OP said he was putting in an upfront rcd would that not cover the disconnection time for all final circuits aswell , saying that i don't think the design is the best but we don't know the resons for the design and there could well be a good reason fot it

true but i cant see why the original installer did this

 

[biospark80]

To complete this issue I've been in touch with Schnieder as it was there MCCB in the panel, they have provided a Vigirex unit with a Toroid and a shunt trip. This is a essentially a earth current monitoring device rated from 0.3-1 amp and the shunt trip attaches to the MCCB to activate the tripping mechanism.

 

[biospark80]

Hi the original installer, has made a huge mistake. I can only assume because he was separating the earth at the load end and creating a TT ( again not sure why as it is a TN-S system, lack of research i think.) he thought that he didnt require the Zs value at the end of the supply cable.

true but i cant see why the original installer did this