Design support needed!

[grumpyjohn01]

Hi All, I have attached (I think) a line drawing of a new install I'm doing. Cost has been paramount and to this end I originally decided to use the Earthing terminal provided ,have a TNCS install, exported from DB1 to DB2 and DB3 just using 30mA RCD protection on sockets and concealed cables, none of the subs are rcd protected. The remote pump is fed via RCBO. However I neglected to recall the requirement for the woodworking shop to be 300mA RCD protected for fire reasons, which I have now realised. Also doing Ze on new supply today (meter went in today) I have wildly differing loops over the 3 phases (L2 being at or about 0.34) suggesting to me that the overheads arent in the best of shape! So should I TT instead? staking down both buildings and putting in an up front RCD ? opinions please!

 

[davesparks]

What are the loop impedance readings for the three phases?
If you are sure of your test results then get on to the DNO to get it fixed.

 

[grumpyjohn01]

In truth I'm not certain as my MFT was being calibrated today so just used the old faithful Robin which gave L1 0.25 L2 0.34 L3 0.28.. they were not steady and those were the best!

 

[davesparks]

A difference of 0.09 isn't exactly wildly varying.
When you say 'old faithful' do you mean it is fully functional and reliable to give accurate readings, or do you mean the one you retired from service years ago and may be u/s?
Test it with your mft when you get it back, and if the results are bad then get the DNO to fix it.

 

[grumpyjohn01]

as I say those were the best results they were not steady over repeated tests readings up to 0.46 were seen, anyhow as you say will take my newly calibrated meter and check. What about the earthing system though, even if the Ze was acceptable would you leave it on TNCS and just add the RCD fire protection?

 

[grumpyjohn01]

Also just cos you report a fault like that to DNO doesnt mean they will do anything...they dont seem to care about the 0.35 thing too much...not round here anyway!

 

[davesparks]

I expect they do care about the 0.35 thing, but are aware that it is only a guideline and not a legal requirement.

If the Ze is acceptable then use it, if not then I'd call the DNO. If they have provided an earth I'll use it unless there is a reason within the regs not to.

 

[Engineer54]

Hi All, I have attached (I think) a line drawing of a new install I'm doing. Cost has been paramount and to this end I originally decided to use the Earthing terminal provided ,have a TNCS install, exported (you're not exporting anything. ...you are ''extending'' an equipotenial zone) from DB1 to DB2 and DB3 just using 30mA RCD protection on sockets and concealed cables, none of the subs are rcd protected. The remote pump is fed via RCBO. However I neglected to recall the requirement for the woodworking shop to be 300mA RCD protected for fire reasons, which I have now realised. Also doing Ze on new supply today (meter went in today) I have wildly differing loops over the 3 phases (L2 being at or about 0.34) suggesting to me that the overheads arent in the best of shape! So should I TT instead? staking down both buildings and putting in an up front RCD ? opinions please!

What do you mean by new supply?? Oh and if it is a DNO overhead supply, it will almost certainly be PME rather than TNC-S...

What's the size's are these DB's and what cable type and number of cores are you intending to use?? What are the cable distances involved.

TT should only be considered as a last resort, that shouldn't be just jumped on as a quick ''get out of jail'' alternative... Nothing to stop you roding the two buildings though, and connecting to the MET and EMT, but don't bother if you are thinking along the lines of 200 ohms Ra's they will need to be under 10 ohms to be of any use to the incoming PME supply

 

[grumpyjohn01]

Thanks for your reply E54, I am going back to job tomorrow with freshly calibrated meter.
New supply as in just installed, I think they describe it as PNB, but we've been down that road already.....they have supplied an earthing terminal. The overheads are separate bare copper and the TX is only 2 poles away as there was already a 3ph supply in the adjacent property so no new overheads have been installed.
The cable sizes have been calculated and have CPC of a size adequate to be a combined bonding conductor.
Was considering rodding but not sure what to expect as its off my turf...the West Somerset red soil is outside my experience!! how deep do you think to get the values you mention? if it can't be achieved..what then? Thanks John

 

[Darkwood]

Back to your penciled design .. Don't be running DB's out of DB's if they are just mcb boards ... partial discrimination at best will be between the front end mcbs and the circuit mcb's not a set-up you want for workshops.

Either have a busbar chamber with HRC fusig swa supply cables to each of the DB2 and DB3 separately or a Power mccb board or budget option front main switch feeding henley blocks and tail out to fused isolators to feed the DB2 an 3.

 

[grumpyjohn01]

Back to your penciled design .. Don't be running DB's out of DB's if they are just mcb boards ... partial discrimination at best will be between the front end mcbs and the circuit mcb's not a set-up you want for workshops.

Either have a busbar chamber with HRC fusig swa supply cables to each of the DB2 and DB3 separately or a Power mccb board or budget option front main switch feeding henley blocks and tail out to fused isolators to feed the DB2 an 3.

Thanks, You are right of course in 'good practice' terms although I wasn't aware of the partial discrimination issue you mention. The boards were specced for their cost efficiency in all honesty as the client wouldnt go for the full industrial setup. The workshop is essentially one man with a lot of toys building fancy kitchens so it didnt really seem appropriate to spend out on BB chambers etc etc!

 

[davesparks]

Just remember there is no regulation which allows you to pick and choose amongst the other regs if the client is as tight as a ducks rear end.

You need to design good discrimination in to the installation.
Regardless of how many people are working in a workshop or whether they are pro or not, if a fault on a machine takes out the submain and thus plunges the workshop into darkness that could be very dangerous.

 

[Darkwood]

Thanks, You are right of course in 'good practice' terms although I wasn't aware of the partial discrimination issue you mention. The boards were specced for their cost efficiency in all honesty as the client wouldnt go for the full industrial setup. The workshop is essentially one man with a lot of toys building fancy kitchens so it didnt really seem appropriate to spend out on BB chambers etc etc!

Formulate a budget plan and express the inconvenience that may occur during fault, been pot luck the upstream mcb is just as likely to trip as the downstream device, you follow regulations for design regarding this area and as your supply is only small incoming you will actually not achieve full discrimination anyway so you do have a little natural leeway.

If your customer goes for the budget option your name is still on the sheet for design but in the real world if its just an inconvenience that may occur and has no safety issues then really your not going to end up in any court for it ... its a private job for a small customer, obviously bigger projects will increase the need to give good design and minimise inconvenience .thus a need to provide a well designed system to limit issues with discrimination but yeah your working to the customers budget - Note though - they usually express what they want to spend on the electrics and not what they can afford, don't cut corners on other regs to appease you customer - the cart should not be leading the horse here so stand your ground on any other matters.

 

[grumpyjohn01]

Thanks fellas, I would never compromise on safety, but there has to be a balance between cost and a slight nuisance that may never happen. Thanks very much for opinions and help.

 

[grumpyjohn01]

Just an update for anybody who is interested, contacted WPD today to report the dodgy L2 reading and their men came straight out. After having confirmed my findings and having a quick check on the nearest pole they went away. Engineer called me a bit later to say that as the reading was under 0.8 they werent going to do anything. At this point I somewhat chucked my toys out of the pram and expressed in no uncertain terms my displeasure, whereupon he reluctantly agreed to investigate further. The men came back and replaced the fuse unit at the TX and lo and behold 0.26 on all 3 phases..Happy days!!

 

[Darkwood]

Cheers for the update .... hope you got all yer toys back