Earth rods to reduce Ze

[davesparks]

So let me get this straight, what you actually want to know is what to do about the fact you have a 200Amp service with a Ze of 0.22 when you need to achieve a Zs on a submain of 0.15?

I have to say it is an interesting question as based on the info supplied it isn't possible.

Off the top of my head I can only come up with splitting the installation into 2 and running a second 125A submain alongside the one you have.

 

[stevethesparks]

You may be in a position to double up rather than replace, if the calcs agree it could be a far cheaper option.

 

[davesparks]

You may be in a position to double up rather than replace, if the calcs agree it could be a far cheaper option.

Double up what exactly?

He has a Ze of 0.22 and needs a Zs of 0.15 for the submain!

 

[D Skelton]

The cables

 

[davesparks]

But the Ze is higher than the required Zs, no matter how big the cable R1+R2 ain't going to be -0.07!

 

[D Skelton]

Ah, I missed that bit. Yeah, I'd go with two 100A supplies then. The DNO isn't gonna be able to help.

 

[davesparks]

More accurately the DNO won't want to help

 

[Darkwood]

I forgot to add; although it is new, the cable will be changed, so the fact that the cable is maxed out will soon not be a problem.

So this has gone from a long winded discussion about the impression you gave with your opening post to a real question of having the cable upgraded etc but as the incomming is already higher than the required Zs it can't be met ...

Run - 2 x 100amp sub-supplys from the 200amp incomer and split the distribution room into 2 supplies ..Zs complies you can re-use one existing cable and just add another......simples

 

[Engineer54]

Shame no-one actually bothered to conduct a Ze test after the new 200A supply was installed. Perhaps then the question could have been asked to the installing DNO, how their 200A service fuses will comply or operate effectively with a Ze of 0.22?? Or do their 200A fuses work on a completely different electrical science/theory?? ...lol!!

 

[Gavin A]

Shame no-one actually bothered to conduct a Ze test after the new 200A supply was installed. Perhaps then the question could have been asked to the installing DNO, how their 200A service fuses will comply or operate effectively with a Ze of 0.22?? Or do their 200A fuses work on a completely different electrical science/theory?? ...lol!!

does sound like a DNO issue if they've been paid to supply a 200A per phase supply, and haven't supplied the earthing required.

 

[wozz]

Was their a figure for Ze provided wit the contract for the new supply?

 

[Kamikaze]

I think the Ze at the intake position of 0.22 ohms is probably wrong and should be double checked. And even if the undersized sub-main cable and the Zs issues are sorted out, I would still not be impressed. There would be 200A fuses in the D.N.O. cut-out, 200A fuses in the main isolator and 200A fuses in the switch room main isolator. Why not just replace every fuse within the system with 200A and be done with it!:angel_smile:

I’m with Sheriff Buford T. Justice on this one :biggrin:. See post No 28.

 

[telectrix]

I think the Ze at the intake position of 0.22 ohms is probably wrong and should be double checked. And even if the undersized sub-main cable and the Zs issues are sorted out, I would still not be impressed. There would be 200A fuses in the D.N.O. cut-out, 200A fuses in the main isolator and 200A fuses in the switch room main isolator. Why not just replace every fuse within the system with 200A and be done with it!:angel_smile:

I’m with Sheriff Buford T. Justice on this one :biggrin:. See post No 28.

I'll second that. the only logical get-out.

 

[Darkwood]

I think the Ze at the intake position of 0.22 ohms is probably wrong and should be double checked. And even if the undersized sub-main cable and the Zs issues are sorted out, I would still not be impressed. There would be 200A fuses in the D.N.O. cut-out, 200A fuses in the main isolator and 200A fuses in the switch room main isolator. Why not just replace every fuse within the system with 200A and be done with it!:angel_smile:

I’m with Sheriff Buford T. Justice on this one :biggrin:. See post No 28.

Usually the customer 200amp fuses will be BS88 Type 2 where as the DNO use Type 3 or 4, the distribution room would not have fuses in it would just be a isolator as the cable is already protected upstream by the switch/fusr in the meter hut.

 

[Engineer54]

Usually the customer 200amp fuses will be BS88 Type 2 where as the DNO use Type 3 or 4, the distribution room would not have fuses in it would just be a isolator as the cable is already protected upstream by the switch/fusr in the meter hut.

Haven't been involved with fuses to any real extent now for a good many years. So what charcteristics will BS 88 type 3 or 4 fuses have over a BS 88 type 2 with regards to maximum Zs values??

 

[Marxman99]

Thanks Darkwood, last night I did have the thought to split the installation. Put to 2no. outgoing 100A switches in the intake cabinet. In the main switchroom i'll have to do some configuration there. I remember reading that in the situation where the Zs fails, an RCD can be used, it can't lower the Ze but it can be used as an alternative safety feature. Probably wont be using the RCD though, but it would be good if I can table options. Make myself seem real technical ; -) Thanks to all again.

 

[Darkwood]

Thanks Darkwood, last night I did have the thought to split the installation. Put to 2no. outgoing 100A switches in the intake cabinet. In the main switchroom i'll have to do some configuration there. I remember reading that in the situation where the Zs fails, an RCD can be used, it can't lower the Ze but it can be used as an alternative safety feature. Probably wont be using the RCD though, but it would be good if I can table options. Make myself seem real technical ; -) Thanks to all again.

RCD'S are fine on a final circuit to meet regulation but to put a 30mA rcd upstream of a 200amp distribution system is crazy it would never hold ...your normal leakage from the complex will probably be higher than the trip threshhold and the regs also say your installation should be designed to avoid situations where a fault on a circuit should not cause inconvenience and problems on other circuits ..especially in your set-up.

Dont forget you still need 1 point of isolation for the whole complex including your 2 x 100amp feeders.. again this is stuff you should know with your training in this field.

Careful design and monitoring of the existing design leakage would be required for higher rated rcd's again nuisance tripping is your enemy here and any downstream fault can take out the complex and can be costly to the customer and yourself so ensure you do all the necessary measurements and calcs prior...Also room for expansion etc Electronic ballast lighting and computers are often regular additions to buildings.

 

[Marxman99]

I considered a single point of isolation to be in the intake cupboard, the existing 200a switch would feed 2no new 100amp switches. RCD on the incomer I would look into but more out of interest, because 100mA RCDs are available....

 

[Darkwood]

Haven't been involved with fuses to any real extent now for a good many years. So what charcteristics will BS 88 type 3 or 4 fuses have over a BS 88 type 2 with regards to maximum Zs values??

The DNO's fusing Zs requirements TBH are their issue if they comply or not but from experience the DNO work to different regs and often in confliction with our own, as is a small supply in the scope of things then full discrimination cannot be achieved and little benefit will be had by derating to 160A customer side.

If the customer has a 200amp (approx' 140KVA) allowance then he should be able to use it all, an O/L will correctly take out the customer fusing first if 200amp BS88's (2) against 200amp BS88 (3) leaving only S/C faults an issue and as I mentioned you can't achieve full discrimination anyway on these small supplies so pointless derating customer upstream fusing.

It would really have to be a massive failure of the supply tails or main feed cable to the dist' room that causes the circumstances where the DNO's may operate first and IMO its not something design can change without derating the fusing massively making them too small for the sites demands, If the massive failure upstream did take the DNO's out its a circumstance Id be happy to get the DNO out.

Yes Im teaching you to suck eggs here and not what you asked but posted for the benefit of the thread not directed at yourself.

 

[Darkwood]

Hypothetical here.... not telling you how to do it.

You have a 140KVA supply
Zs can only be achieved on your side if you fusing to BS88(2) is 125amp or less
You can run 2 x 100amp supplies but issue is they need a common isolation upstream
Fitting a 200amp SW/F upstream would not comply because of Zs failure as 0.15 cannot be acheived.

What will happen in an earth fault then if your Zs is 0.22 instead of 0.15... your cabling could be subject to thermal damage due to the extended duration of the fault, so what are your solutions here?

Edited ... saw a flaw in my suggestion .. its quoted below though if you want to browse.