New place, rat’s maze wiring to workshop.

[Guineafowl]

Distance from service head to shop, in wiring length, probably 30-40m.

Problem 1: final Zs is 125 ohms. Fault is probably in JB1 or JB2. I can hopefully sort that, but will need a ladder to get there.

Problem 2: Three DBs, all remote from the workshop, give me a discrimination (selectivity?) problem.

Plan: Sort earth fault, then see if I can maintain 40A type C MCBs up to a new DB in the workshop, as I’ll need at least a 32A type C for the phase converter, let alone the normal sockets.

Diagram (apologies if it’s unclear - it really is that much of a mess) shows existing in blue, changes in red. I’m hoping the 6mm2 is plenty for a 40A supply, but should I just make sure the loop impedance is below the 0.55ohm for a 40A C MCB?

 

[gaz1]

query here is your shed inbetween your house and the workshop

how far is it from your shed to the house and how far is it from your house to the workshop

id rather run a good wire to the workshop in one run

if the shed is near the workshop wire off from the db at the workshop

 

[Hotstuff]

No electrician would recommend that you add to this installation; by your own admission “it really is that much of a mess”.

As Gaz1 suggested, run a new sub main from DB1 and design the circuit properly to suit your requirements.

You will need to consider the fuse discrimination between your sub main MCB and the sub circuit serving the phase converter.

It’s unlikely that you will get the Zs down below 0.55 Ohm at the furthest point of its respective sub circuit; especially if you use the cables indicated on your drawing and the distances mentioned.
Your alternative would be to provide RCD protection to the circuit.

If the workshop is remote from the house you need to consider using a TT earthing system to prevent risks associated with earthing system potential differences.

I couldn’t help notice that you have scheduled various non-compliances within your post and drawing but not raised them as concerns.

Type C MCBs are not really suited to domestic installations. Replacing the MCBs serving DB1 & DB2 with 40 Amp type C devices may not offer the best protection.

You need to consider fuse discrimination and loading at the fuse boards.

Please be careful

 

[dobbslc]

I'm wondering if the shed and workshop actually are on a TT system with an earth rod hidden somewhere at the end of the circuit with a bad connection or just poor ground conditions, what sort of soil are you on? Clay or chalk are pretty well at the extreme ends of good and bad for earth electrodes.
Are there any RCD's in the dist boards?
If the wiring is in decent condition maybe you could just replace the boards to ones with RCD's in them?
Any photos?

 

[Guineafowl]

Thanks. New sub main certainly an option.

I’m still moving in, so will have another look today when I shift some more boxes.

Don’t worry, I’ll be guided by the numbers when selecting MCBs. What I don’t want is a breaker tripping in the house when I’m up at the workshop. Also don’t want fire/shock, so I’ll be careful.

I’ll have a look for an earth rod, but I’d expect better readings than that anyway - the transformer’s pretty close. Soil is nothing extreme.

 

[Hotstuff]

I'm wondering if the shed and workshop actually are on a TT system with an earth rod hidden somewhere at the end of the circuit with a bad connection or just poor ground conditions, what sort of soil are you on? Clay or chalk are pretty well at the extreme ends of good and bad for earth electrodes.
Are there any RCD's in the dist boards?
If the wiring is in decent condition maybe you could just replace the boards to ones with RCD's in them?
Any photos?

Do you feel that 125 Ohm is a poor impedance for a TT earthing system?

If that is the case, then from the OP’s description, it’s likely that the service earth system is present or connected to the TT (true earth). I wonder if that could present a possible hazard associated with potential difference?

 

[Guineafowl]

This thing is the most hungry device I’m planning to use. It needs a 32A C MCB:

Digital Phase Converter 5.5Kw/7.5Hp 240V in 400V Out - JFK Electrical

5.5 Kw / 7.5 Hp 240 Volt Digital Phase Converter Input Volts 240 Single Phase ( Supply 32Amp type C Mcb ) Output Volts 415 Volt 3 Phase ( No Neutral ) Dimensions L 280mm x W 145mm x d 180mm Weight 4.5 Kg Change your Garage/Workshop from single 240 volt to 3 phase 415 … Continue reading Digital...

www.jfkelectricalni.com

I have an old spindle moulder with a big 3ph Dahlander two-speed motor, 4hp on high speed, so I think it’s ^ needed.

As you can see, it’s not the running current, but the inrush that might cause selectivity problems. If I have a 32A C at the point of use, is a 40A B upstream of that going to behave itself?

 

[Onoff]

Take off and nuke it from space. Then start again!

Rat's nest is the word. Just putting it out there...could you consider making each DB it's own TT island with a Type S 100mA up front in each. Each DB provides earth protection to the onward cable but you stop it short before the next DB. Each DB has a local rod. The "only" selectivity issue is then between the 100mA units.

 

[Guineafowl]

Take off and nuke it from space. Then start again!

Rat's nest is the word. Just putting it out there...could you consider making each DB it's own TT island with a Type S 100mA up front in each. Each DB provides earth protection to the onward cable but you stop it short before the next DB. Each DB has a local rod. The "only" selectivity issue is then between the 100mA units.

View attachment 464404

All grist to the mill, cheers. Things might change, but at the moment I’m not that bothered about exporting the TNS earth. The building is no more than 30m from the house, and that in turn is a few metres from the transformer.

100mA RCDs might be a good option for protecting the cable, then 30mA at point of use. Sounds good.

 

[Onoff]

100mA RCDs might be a good option for protecting the cable, then 30mA at point of use. Sounds good.

Not just 100mA RCDs but time delay, type S ones, like this. Put a standard 100mA RCD before a 30mA one and you can still have selectively issues if you you don't have the time delay element.

https://flic.kr/p/2pVWg7k

You know your "ideal" is a number of switched fuses at source fed from Henleys straight after the meter. Then a separate feed to each outbuilding.

That and a stock of head fuses!

 

[Guineafowl]

Finally got the roof ladder up to have a look - earth disconnected at JB1. No idea why.

Zs-db for DB3 is now 0.53 ohms so in theory could be served by a 40A C breaker. I may still run a new 6mm2 sub main, easiest from an S-type 100mA RCD in DB2, all the way to the workshop, bypassing DB3.

 

[Dave-sparks]

All grist to the mill, cheers. Things might change, but at the moment I’m not that bothered about exporting the TNS earth. The building is no more than 30m from the house, and that in turn is a few metres from the transformer.

You aren't exporting it anywhere, you're extending it. If it's TNS then there's nothing to be gained by creating a sperate TT system for the outbuildings, you'll be sacrificing a very good earth connection for a much poorer one.

100mA RCDs might be a good option for protecting the cable, then 30mA at point of use. Sounds good.

100mA time delayed RCDs would only be required if using a TT system and you are unable to get a suitably low Zs for MCBs to operate correctly.

 

[Guineafowl]

You aren't exporting it anywhere, you're extending it. If it's TNS then there's nothing to be gained by creating a sperate TT system for the outbuildings, you'll be sacrificing a very good earth connection for a much poorer one.

Ah, I thought exporting = extending. Anyway, as you suggest, I’ll keep the TN-S earth. My favourite type of earthing.

100mA time delayed RCDs would only be required if using a TT system and you are unable to get a suitably low Zs for MCBs to operate correctly.

See Onoff’s comment above. I’d consider a 100mA normal type at DB2, for the cable run, then 30mA at point of use (workshop). I definitely don’t want a 30mA job tripping all the way at the house when I’m in the workshop…

 

[sparkysy]

If you do put in an upfront RCD I would use a time delay type, that and using double pole RCBO's at the workshop end!
At least that will give you some discrimination between the RCDs and which trips first.

 

[Dave-sparks]

See Onoff’s comment above. I’d consider a 100mA normal type at DB2, for the cable run, then 30mA at point of use (workshop). I definitely don’t want a 30mA job tripping all the way at the house when I’m in the workshop…

When you say 'for the cable run' what do you mean? Is there a particular type of fault or scenario you have in mind that you want it to protect against?

If it's TNS then the 100mA is unnecessary and if it is of the non time delayed type it will almost always trip at the same time as the downstream 30mA one.
Assuming it will be armoured cable properly installed then no RCD protection will be required for the submain.

 

[Guineafowl]

When you say 'for the cable run' what do you mean? Is there a particular type of fault or scenario you have in mind that you want it to protect against?

If it's TNS then the 100mA is unnecessary and if it is of the non time delayed type it will almost always trip at the same time as the downstream 30mA one.
Assuming it will be armoured cable properly installed then no RCD protection will be required for the submain.

Between DB2 and JB1 there’s only 6mm2 T&E, and I can’t follow it all the way along its run. Just in case it gets drilled into.

If I start anew with SWA then yes, no RCD.

 

[Dave-sparks]

Finally got the roof ladder up to have a look - earth disconnected at JB1. No idea why.

Zs-db for DB3 is now 0.53 ohms so in theory could be served by a 40A C breaker. I may still run a new 6mm2 sub main, easiest from an S-type 100mA RCD in DB2, all the way to the workshop, bypassing DB3.
View attachment 464916

The 3 40A MCBs and the 32A MCB in DB4 will not discriminate and it's more likely that the MCB in DB1 will trip than any if the others if there's an overload or a fault.
MCBs in the standard range of 6-63A generally won't discriminate with each other on faults, they will generally discriminate as expected for an overload.

You'll have a better chance of achieving discrimination if the submain is protected by a fuse rather than an MCB.

 

[Dave-sparks]

Between DB2 and JB1 there’s only 6mm2 T&E, and I can’t follow it all the way along its run. Just in case it gets drilled into.

In that case you can't use a 100mA RCD, the regulations require it to be a 30mA non-time delayed RCD as it is intended to provide protection against an electric shock becoming fatal.
With a 100mA RCD you will be dead before it disconnects.

 

[Guineafowl]

In that case you can't use a 100mA RCD, the regulations require it to be a 30mA non-time delayed RCD as it is intended to provide protection against an electric shock becoming fatal.
With a 100mA RCD you will be dead before it disconnects.

Ok then. I need a proper look at the run from DB2, not just a peek while moving in boxes.

I think the 6mm2 T&E is either easily visible or within prescribed zones. Will check.

 

[Hotstuff]

Just a thought and not intended to teach anyone to suck eggs; do consider the possibility that although you have TNS earthing system at your service head, it could (most likely) be terminated to a TNC-S supply network.