ZE+(R1+R2) Sub-mains?

[rob121]

Morning All,
Just a quick one thats got me stumped. Got my NIC assessment coming up so wanted to run this one past you! Im doing a barn conversion, basically dist circuit from customers house to CU then obviously onto final circuits. Now ive taken all the supply characteristics from inside the house, where the cut out fuse is. Now when ive done all the Zs tests, ive just used measurement, including for lighting circuits. What im wondering is if the assessor whats me to CALCULATE the loop impedance, how do I go about it, do I need to take into account the dist results? Bit of additional info, its a TT system, ive installed seperate earth rods at all dist boards (dist circuit supplies garage, barn conversion, then play house).

One more quicky, just wanted to confirm zs results, being a TT system theyre obviously higher than the tabulated results, would I be right in saying that as long as theyre under 200ohm thats acceptable? Results are actually around the 12ohm mark, which if im right in comparing to the 200ohm limit is pretty reasonable?


Cheers again folks!

 

[telectrix]

i wouls say that your Zs for the barn would be calculated as (R1+R2)+ Zdb, where Zdb is the EFLI at the barn CU and takes into account the Ze at ther main CU. the 12ohms is very good fot TT. bear in mind that your max. Zs entered on the EIC will be 1667.

 

[T&Earth]

Hi Rob 121
As I understand for Sub board the Zs becomes Zdb, which should be Ze of main board + resistance of supply cable then R1+R2 of final circuit I think. As you have two earth rods then the supply cable or core that you are using for CPC should only be terminated at the main board end.
Hope this helps but I'm not 100% sure re Zs at sub board.

 

[Deleted member 26818]

As you are issuing an EIC, you will be expected to measure and record the R1 + R2 of each circuit you install.
As such calculating Zs at any point will be quite straight forward.

 

[telectrix]

he would only termimate the cpc at 1 end if the earthing was TN at the main, and TT at the sub. from what i read of the OP, it's TT at both ends.

 

[rob121]

Thanks for the replies,
I thought it would be a case of adding the measured resistance of dist circuit to the Ze (actually I believe its Ra for TT thinking about it?) then obv adding the resistance of the final circuit onto that.
Yes its a TT system so have inter-linked all the earth rods (theres actually 4 of them, house, garage, barn, playhouse). I did read about not connecting rods to a TN system on here before but cant remember why, something about being near factories on with PME I think. I shall have to dig that thread out!

Thanks again for the responses,

Rob

 

[malcolmsanford]

Morning All,
Just a quick one thats got me stumped. Got my NIC assessment coming up so wanted to run this one past you! Im doing a barn conversion, basically dist circuit from customers house to CU then obviously onto final circuits.As this is a TT system are you going to protect this sub main with 100mA RCD protection as there is a good chance that your disconnection times will not be met

Now ive taken all the supply characteristics from inside the house, where the cut out fuse is. Now when ive done all the Zs tests, ive just used measurement, including for lighting circuits. What im wondering is if the assessor whats me to CALCULATE the loop impedance, By Loop impedance I assume you mean R1 + R2. Yes you will need to do either them, or R2 for you Schedule of Test Results, as it is a new installation I would do the R1 + R2

how do I go about it, do I need to take into account the dist results? Is that the Distribution cable results?

Bit of additional info, its a TT system, ive installed seperate earth rods at all dist boards (dist circuit supplies garage, barn conversion, then play house).

One more quicky, just wanted to confirm zs results, being a TT system theyre obviously higher than the tabulated results, would I be right in saying that as long as theyre under 200ohm thats acceptable? That is the BS 7671-2008 values for the earth electrode, see note 2 of table 41.5 in the BS 7671-2008. Your ZS values for your 30mA RCD for additional protection will be 1667 ohms. The ZS/Zdb for your distribution Circuit if your using a 100 ma RCD will be 500ohm,

Results are actually around the 12ohm mark, which if im right in comparing to the 200ohm limit is pretty reasonable? To be honest I would like to get that lower,as I said above you can in fact have a Zs value of 1667 ohms to operate your RCD, but it is always best to get the rod down as low as possible. but yes it is acceptable


Cheers again folks!

Hope above helps

 

[rob121]

Thanks Malcolm, seems your in the same neck of the woods as me. Im in Ashford, Kent.

Yes the whole installation is covered by a 100mA RCD. I have made a note of the cert as the RCD is not type S. One other issue ive had to record on my results is the playhouse which is supplied from my unit does not have an RCD at its CU. This means the playhouse is only covered by the 100mA at the house. Ive made a note of it, but am I responsible for this, i.e shouldnt connect the p/house until a 30 mA is fitted? The playhouse CU supplies has one 32A Ring and one 6A Lighting circuit. The sockets are the issue as theyre likely to supply outdoor equipment.

Many thanks,

Rob

 

[telectrix]

if the lighting in the playhouse is not buried less than 50mm deep in walls, then you can RCBO the sockets. if you want to RCD all the playhouse, fit a garage board with RCD and 2 MCBs.

 

[rob121]

Replacing the playhouse CU for one incorporating a 30mA RCD isnt an issue. Its whether the customer wants to pay for one (they most likely wont mind paying for one but theyre on holiday for a month). Just wanted to confirm I wasnt responsible for the installation because Ive connected it (same as when upgrading a CU)?
Thanks for the reply
Rob

 

[SPARTYKUS]

If you energise something then you have become responsible for it, or so I am lead to believe. The point being if you energise it and it wasn't safe to do so, its your fault.

ps which is what annoys me about builders that say "can't you just power it up" or "you cant find anything wrong with this one there's no money in the pot"

 

[rob121]

Yes thats what I was thinking. For now I shall disconnect the supply until ive looked into it further. Yes unfortuantely even in my short amount of time ive had builders/customers pressuring me to cut corners. Remember one builder telling me it was labour only and he would supply the parts. Turned up to find second hand T+E in old colours, which was left over stock and only connector strips instead of JBs. Needless to say I quickly took a trip to the nearest wholesalers! I dont think I ever received the money for the materials either. Live and learn ay!

 

[telectrix]

are these the same builders that would test a cable under load by dumping a few cwt of sand on it?

 

[SPARTYKUS]

are these the same builders that would test a cable under load by dumping a few cwt of sand on it?

thats the bloke you know him too then.

yeah had a bit done for me 2 years back by a plasterer trying to speed job up, ran a bit of T+E for me. right size, wrong colours

 

[William Irons]

if you have an installation and its TT, an earth electrode of say 50 ohms you take a reading at the dist board and its 10 ohms say so you may say why is it so low ............maybe a parallel earth path from a steel beam or water or gas pipe therefore all your zs readings are going to be lower than your ze earth electrode reading. Is there anything wrong with taking the lower reading at the dist bd to calculate R1+R2 from each final circuit Zs reading ( asking this as mft1730 calculates this by taking Ze from each circuit Zs)