Second CU in domestic workshop - some advice please?

[lawsonium]

Hello,

If I wanted to put a second Consumer Unit in a domestic workshop with the following criteria, what cable would you advise for the extension.

Remote CU would be 50m away from the main board. The main board is not in a house but a separate garage, is a dedicated supply and has it's own earth.
Lights and sockets only, calculated to be no more than 10kW total (though never used at full capacity).
Switched fuse or RCD to protect the extension on the main board side of the cable run.
The workshop where the remote CU will be TT'd with it's own Earth.

My VD calculations suggest 16mm² SWA would be fine for the extension to the remote CU with this situation.
VD of 5.43V (2.4%) at a max of 43.5A
I don't think I can get away with 10mm² due to the VD and this having lighting circuits.

Second question is about the usage of 2 core SWA instead of 3 core?
I don't need to export the earth as the workshop will be TT'd and so I would just earth the armor at the main board. Thus the thought that 2 core is fine to use in this scenario (easier to work with and slightly cheaper).
Reason I ask is that online calculators suggest that the VD calculations are different between 2 core and 3 core SWA cables - i.e. there is greater VD with the 2 core than the 3 core SWA cables which doesn't make any sense to me but then, I'm not an expert. Thoughts?

 

[telectrix]

you could exceed the 3% for VD on lighting is you fit LED lights, as these will happily run on <240V. unless the supply voltage is at the lower end of it's tolerance (216V), your added volt drop might have some effect. personally I'd go for 10mm cable, assuming that you have no intentions of increasing the load above what you stated. and why TT when you can use the existing earthing arrangement? If you have extraneous metal in the workshop though, you'd need 3 core SWA to provide a suitably sized cpc/bonding conductor.

 

[radiohead]

@ Mods
Presumably there is an offensive word blocker in place on the forum. Would it be possible to include the words 'EXPORTING/EXPORT' when used with the term EARTH in this bit of software?

 

[pc1966]

Reason I ask is that online calculators suggest that the VD calculations are different between 2 core and 3 core SWA cables - i.e. there is greater VD with the 2 core than the 3 core SWA cables which doesn't make any sense to me but then, I'm not an expert. Thoughts?

Cable has both resistance and inductance, and they both vary a bit with frequency as well (look up "skin effect") so the different size/geometry of the 2 and 3 core cables has a slightly different impedance at 50Hz AC even though the DC resistance of, say 16mm copper, is the same in both cases.

Usually it is not big enough to worry about. When you get to larger cables they normally tabulate the R and X terms separately so you can do impedance calculations more accurately to get the voltage drop & PFC, etc, from a known supply.

 

[lawsonium]

Appreciate the responses. Thank you.
Yes, it will only be LED lighting. Strip in the workshop but some pretty hefty 200w and 100w floods outside and some lower wattage bulbs and strips in adjacent stables and stores, all LED though.

Should I be worried about the supply voltage being low? It's direct from the transformer on a nearby pole. Best way to check? Simple volt meter test close to the existing CU?

From the research I have done, I was thinking about a local earth in the workshop due to the distance from the pre existing earth arrangement. Is this not a factor?
What would be the deciding factor in this?

With some trepidation, may I ask what's "offensive" about refering to "exporting the earth", what term should I be using here?

I'd much rather employ an electrician but the ones I have spoken to aren't keen once they know it's a farm situation. I may try some others first (may have to be industrial contractors rather than domestic people) but thought I'd run my plans through this forum first. Helps to know what you are talking about when discussing anyway.

Many thanks.

 

[pc1966]

Generally speaking additional earths are a good thing.

However, if the supply is TN-C-S and you have an earth rod or any extraneous conductive parts to bond (metal water pipe, steel structure, etc) then the minimum size of bonding wire all the way back to the Main Earth Terminal is larger to allow for the risk of a sustained high earth current in the event of a PME fault.

If your supply cable is not more than 35mm (which covers most installations at 100A or less) then the minimum size for earth bonding is 10mm and (as mentioned above) that can be a reason for going for 3-core SWA as the armour of the cable is not as conductive (e.g. 10mm 2-core has about 26mm of steel, which is equivalent to around 3.2mm of copper and so is not enough for bonding where TN-C-S is in use).

You mention it is a farm with a local transformer, but what matters is how the supply is delivered to you.

Within any UK installation it has to be TN-S (separate earth & neutral) and only the DNO is allowed TN-C (combined E&N, also known and CNE, PEN or PME) as the risk of a fault leading to dangerous voltages on apparently "safe" earthed objects is high. So the common arrangement is a combined N&E conductor to the DNO's cut-out/fuse holder, then separate inside the installation.

If you have a photo of the incoming supply arrangement then folk on here should be able to tell what it is.

The "offensive" aspect alluded to is really the expression is in common use but is wrong.

 

[radiohead]

With some trepidation, may I ask what's "offensive" about referring to "exporting the earth", what term should I be using here?


My post was intended to be 'tongue in cheek' but reflects the frustration many electricians feel over the persistent lack of understanding over TNCS earthing to remote buildings. 'You cant export the earth' comes up continually on forums and even in wholesalers despite there being no basis for the view and it's exponents never being able to quote the relevant regulation. It is a myth that just wont go away. No offence intended but it's best to use information in Bs7671 and other relevant documents rather than just believe what you've heard.

 

[Strima]

As you have mentioned it's a farm there are certain requirements within BS7671 for the protection of livestock, this is covered in section 705.

What is the purpose of the workshop? Is this purely for machinery or will there be livestock in it at times? This will have a bearing on how the installation is designed.

 

[lawsonium]

@radiohead I wasn't offended and am not setting out to offend either, I just want to reference things by the correct terminology. So, instead of "exported", what's the most effective way to reference making use of the third core in such a setup to take the earth from the main board?

Ultimately, I have no "opinion" on any of this, only a desire to get it right. I have reached out to an old friend who's husband is an electrician. Hopefully they will either be interested in taking on the work or, at the very least, doing the design. I'm not a "customer"/"client" who minds paying for expert services. I am also not work-shy and incapable.

@pc1966
I will take a photo of what's pre-existing (all of which I intend to upgrade as it's pretty elderly). So would appreciate the lesson here.

There are no metal water installations in any of the buildings and no gas.

I must say, I am confused by the reasons there is such a heated debate over taking the earth from the supply in a remote context since it's hardly difficult or expensive to introduce a new earth in said remote location.

@Strima
No livestock, just machinery, feed and equipment. This intended supply will also be serving external lighting around the building. No high powered equipment, just occasional hand tools (hand drills, angle grinders) and things like battery chargers etc.

 

[radiohead]

@radiohead I wasn't offended and am not setting out to offend either, I just want to reference things by the correct terminology. So, instead of "exported", what's the most effective way to reference making use of the third core in such a setup to take the earth from the main board?

Ultimately, I have no "opinion" on any of this, only a desire to get it right. I have reached out to an old friend who's husband is an electrician. Hopefully they will either be interested in taking on the work or, at the very least, doing the design. I'm not a "customer"/"client" who minds paying for expert services. I am also not work-shy and incapable.

@pc1966
I will take a photo of what's pre-existing (all of which I intend to upgrade as it's pretty elderly). So would appreciate the lesson here.

There are no metal water installations in any of the buildings and no gas.

I must say, I am confused by the reasons there is such a heated debate over taking the earth from the supply in a remote context since it's hardly difficult or expensive to introduce a new earth in said remote location.

@Strima
No livestock, just machinery, feed and equipment. This intended supply will also be serving external lighting around the building. No high powered equipment, just occasional hand tools (hand drills, angle grinders) and things like battery chargers etc.

A minimum CSA copper conductor of 10mm applies to PME installations, so if a remote building with extraneous CP's was fed by a 6mm distribution circuit it would be necessary to run a separate 10mm main bond which would make a TT more practical. If however there was no main bonding requirements, or a 3c 10mm distribution circuit was installed which would serve both as a cpc and main bond why would you want to disconnect a perfectly good earth and install another one?
This myth is only ever applied to outbuildings, if you think about it external lighting, sockets, electric gates etc are no different in terms of a hazard from a broken neutral than a shed, and yet this argument is only ever applied to the latter.
Where an outbuilding is fed from a common supply to the main building the earthing system is not being 'exported', it is simply being extended to an addition to the existing installation....the same as it would be to any addition.

 

[Strima]

No livestock, just machinery, feed and equipment. This intended supply will also be serving external lighting around the building. No high powered equipment, just occasional hand tools (hand drills, angle grinders) and things like battery chargers etc.

That makes life a little easier. If this distribution is supplying circuits that are agricultural or horticultural in nature then they must comply with part 705.

As stated by @radiohead if there's no need for main bonding then there's no reason why the TN earthing system cannot be used. You could still have a water/gas supply into the building as long as they don't have an earth potential, i.e. all done in plastic.

 

[lawsonium]

This is the very tired, old supply.
The earth cable on the right of this photo runs outside to an earth spike.
The earth cable you can see going off to the left is completely unnecessary, it attaches to the only tap in the building which is directly attached to a plastic pipe and nothing else. The tap isn't even connected any more and will be removed. I can only imagine it was done for effect, who knows, before I owned the property.

Closer look at the meter

Closer look at the "consumer unit" lol.

Obviously someone decided to bypass the original RCD or fused switch at some point in the sorry past of this property. All needs renewing, and will be soon hopefully.


Looking at this setup, there is no DNO derived Earth that is used beyond the meter. The CU is only earthed from the local earth spike. What's your assessment of this and the implications for my planned extension of the supply?

Regards.

 

[radiohead]

A potentially dangerous installation at present. If there is an earth electrode it is likely that the Ze will be too high for ADS without RCD protection. It's possible the RCD which should be in the enclosure was removed because it kept tripping due to a fault.....even more dangerous.

 

[Strima]

You need to get that looked at sooner rather than later. A TT system is fine but you need RCD protection as above.

It might be worth enquiring with the DNO about a potential change to TNC-S.

 

[pc1966]

As above, the workshop extension is the least of your worries!

That looks very dodgy, and if it is missing an incoming delay RCD then it is a fatal accident waiting to happen. Really, you must get a professional in to look at this.

A commercial/industrial electrician who is happy to deal with farms is a Godsend - they should be able to resolve the incoming supply arrangement. But being frank with you it looks like you need a new CU with an incoming delay RCD (100mA or 300mA) as well as some RCBOs or similar to provide additional protection for socket outlets, etc.

Don't put this off - it is a serious business!

 

[lawsonium]

Thank you everyone. I appreciate your concern and completely agree. Hence why I prefaced this by saying I intend to replace it all (at the same time as doing the extension). I see this as a single project. I'm aware that everything you see in the photos needs replacing.
I have two new consumer units waiting to go in. One here and one in the remote workshop.

The question I have at the moment, based on this conversation, is whether I need to change this TT earth configuration to TNC-S or not and if that will require the DNO before I or an electrician replaces the CU and installs the extension?
As it happens, the supply at the cottage is TT but it has a newer CU and a functioning main RCD.

Have spoken to my friend and her partner 'isn't taking on any more work at the moment' (might be code for 'not this kind of work though' lol).
I have two other local names to try tomorrow but being out in the countryside options are limited.

 

[pc1966]

You choice of earth is ultimately limited by the DNO.

Only the DNO can offer you a supply earth, and if it is available then it is usually very reliable.

Otherwise (and common in rural areas and in Europe) you are going to be TT and then need to have both a decent earth (no more then 200 ohms for up to a 100mA RCDs) and to have working RCD protection for all circuits.

Traditionally that was in the form of the incomer (in place of an isolation switch) but some go for all-RCBO boards. My own choice would be to have both the delay RCD incomer and RCBOs as then you have no single point of failure.

To get an earth below the 1-1.5 ohm or so needed to clear on faults to 32A MCBs is usually impractical unless you have major steel foundations in a reasonably damp soil. So then you depend on the RCD to clear any fault to earth.

If in doubt ask nearby farms, etc, who they use for any electrical work.

 

[lawsonium]

No major steel foundations but plenty wet ground lol. By the look of the supply, there is a supply earth available but just hasn't been used. Will have to call the supplier to confirm I guess.

If we make an assumption though that the supply will remain TT earthed, am I right in saying that it will be more important to have a new earth to TT the remote workshop rather than taking the earth from the existing TT earth?