Sub main design without measuring Ze (EDITED TITLE)

[NozSpark]

If you've read my post about the worshop leaky roof then you'll know that I have very tight constraints with turning the power off... basically I can't except for planned work and this is causing me a little bit of difficulty..

Right enough flannel,,,

It's a TNS supply and I'm going to replace the existing sub-main with SWA via a 60A BS1361 metal clad switch fuse... so how do I go about calculationg the Zs?? in my mind I'll have to use the guidance Ze of 0.8ohms..

now this is now causing me a little problem in that the guidance Ze is already above the max Zs of the fuse (which is 0.7ohms), so I guess I'll have to design RCD protection into this sub-main,,

My current thought is that I'll have to put a 80A 100mA 'S' type RCD in the switch fuse instead of the main switch,, what do you recon??

 

[Sidewinder]

Use BS88's not 1361's for a starter mate.

Have you a Ze at the moment?

Have you a Zs for the origin?

Don't go as low as a 100mA unless you have to.

Work on your existing Zs at the origin then work your Zs out @ the termination.

Remember in ind/comm you often have lots of steel around!

You can use structural steel as your earth rod in a TT remember.

Use the <50V rule unless you need the rcd for additional.

This way you could get away with a 500 even?

Try to do some discrimination calcs on your downstream cpd's.

Off up the hill now night all.

 

[NozSpark]

Use BS88's not 1361's for a starter mate.Book in van I'm afraid,,, nearly put BS88Have you a Ze at the moment?No Ze at the moment and it'll be difficult to arrange for power off (except for the job)

Have you a Zs for the origin?That was something I was thinking about

Don't go as low as a 100mA unless you have to.Fairy snuff, prob go to 300mA 'S' type then

Work on your existing Zs at the origin then work your Zs out @ the termination.

Remember in ind/comm you often have lots of steel around!No extraneous steel work in this building,, brick/block construction

You can use structural steel as your earth rod in a TT remember.Yep,, I've done this in the past

Use the <50V rule unless you need the rcd for additional.

This way you could get away with a 500 even?

Try to do some discrimination calcs on your downstream cpd's.

Off up the hill now night all.

Mine red

Cheers.. I'll have a better look at it tomorrow

 

[a1]

what do you recon??

I reckon, as you are an elder member, you know you should put a more descriptive title to your thread in order to help a great deal with SEO.

I thank You!

 

[NozSpark]

There you go Admin,,,

just like you asked

anyway, what's all this elder member stuff???? are you saying I'm old???

---------- Post Auto-Merged at 13:46 ---------- Previous post was made at 13:30 ----------

Who hoo,,

Just been back there this morning and measured the Zs at origin, PEFC and PSC

Definately TNS

Zs = 0.12 ohms

PEFC = 0.94kA

PFC = 2.3kA

so it seems that I will get away without having to RCD the submain...

Final circuits will have RCO protection though

Using 17m x 16mm 3c SWA fed from a 63A BS88 metal clad switch fuse my calculated readings are

volt drop = 2.38v

Zs at sub DB = 0.166 ohms (with 1.2 correction factor)

Oh,, and I don't know why I put it as a BS1361 before because I have got a price for a BS88

 

[Sidewinder]

Noz,

3ph or 1ph?

Oh & yes I am still up!

Got to go to bed... sleepy, tired..., ooh, a new post hang on!

 

[NozSpark]

1ph mate

final circuits are 4 x 16A socket circuits and 1 x 6A lighting

TBH for as long as I've known the fella the rear unit has been empty... got to be 4 or 5 years now

 

[Sidewinder]

OK, 3c swa ok, parallel the armour though.

 

[NozSpark]

Of course mate... always do that's why it's 3c

Only 2c if it's TT!