To TT or Not TT thats the question !

[mort2376]

Morning Peeps.

Inherited a storage unit site and they want the electrics looking at as an rcd keeps tripping and taking half the site out.

They have multiple boards fed via swa cable.

The zs on some of the boards is very poor especially on the long runs. There's not much power being drawn only led lighting and the occasional socket.

The incoming is a Tns system with solar and batteries.

There is no way of running additional parallel earth cables as nothings in ducts it's all buried. So my question is would you just add earth rods to the existing boards to help lower the zs readings or would you TT those boards and have them separate from the incoming earth ?

Is it within regs to have different earthing systems within one installation. I'm sure it is with garden sheds being TT when on a long run from house.

I'm only just back of my hols so brain isn't fully back with it yet, so thought I'd consult the hive mind for opinions before I sit down with the regs books and guidance notes.

 

[Murdoch]

Whats the earthing arrangement got to do with the RCD tripping?

 

[mort2376]

That's a separate issue. They have used 30ma rcds as main switches on the sub boards so if a fault occurs anywhere the sub board goes off along with the local one.

I'm putting new sub boards in with just main switches as all cables to boards are swa. I'd just change the switches, but obsolete boards plus no covers !

 

[UNG]

If you put rods in are you trying to hit a low resistance value or what appears to be the norm these days just knock a rod in and lash an earth onto it

 

[mort2376]

I'd like to get Zs values to comply. As the swa are not going to be rcd protected they have to get to the correct values at the sub boards.

From there there may be rcbos on final circuits or mcbs if going to next board.

That's why I'm not sure about just adding the rods or making the entire system TT.

If I make it TT then Zs values don't really matter as 1664 is achievable with wet string ! But it means more rcds on circuits and it won't be easy as it shows already to provide discrimination.

That's why my gut feeling is just add rods at the sub boards to help to bring it within values, the boards are at least 10m apart for rods spaced that distance shouldn't cause any problems interfering with each other.

 

[Murdoch]

Are the SWA cables terminated properly at both ends?

 

[mort2376]

Yes but old swa. Most using swa as the earth, which is fine but from mains to furthest board is probably 50m going via 3 sub boards. All in 16mm 4core swa.

I'm amazed that the volt drop isn't worse, but it's only really running lights in the units at that end of the yard.

Just to add to the story it's an ex farmer which explains the state of the boards at the moment.

Pics below are today's initial exploration. All photos are as found.

 

[Phoenix]

First of all is it Guarenteed TNS, or does it just present as SNE but possibly combined at somewhere on the way back to local TX.

Remember that you can meet disconnection requirements with RCDs, even on TN systems if required. You are able to have at at least three levels of discrimination between RCD devices, You can have standard 30mA Devices on final cirucits, 100mA Time delay devices on submains, and an adjustbale vigi type unit on the main incommer if need be.

But if Zs values are only a little on the high side, have you considered BS88 fuses might be a little bit more forgiving than breakers?

 

[binky]

farmers mess! Probably safer to TT the lot. If I remember correctly theres something about sub mains and 300mA RCD for farm buildings - reliablity with fire protection, in the regs. havnn;t got my book to hand to have a read.

 

[mort2376]

It's no longer a farm it's separate hand built sheds for storage, so no livestock or anything like that to worry about.

I'm off tomorrow as the van is in for its mot a d service so am going to plunge into the regs book then.

 

[SPECIAL LOCATION]

Yes but old swa. Most using swa as the earth, which is fine but from mains to furthest board is probably 50m going via 3 sub boards. All in 16mm 4core swa.

I'm amazed that the volt drop isn't worse, but it's only really running lights in the units at that end of the yard.

Just to add to the story it's an ex farmer which explains the state of the boards at the moment.

Pics below are today's initial exploration. All photos are as found.

Are you sure your poor Zs values are just down to cable lengths and not a few lose joints somewhere along the circuit(s)?

i.e. Have you done any calculations to compare your test readings with the ohms per meter from the tables in the regs?

e.g. from my calcs 50m of 16mm CSA conductors if R1 & R2 are both 16mm would only be 0.12ohms. (2.3x50)/1000 = 0.115
I can't see how that would have a massive impact upon Zs unless your Ze is way overboard?

Not sure what the calcs work out to if using the SWA armour as the CPC..
But I don't think it makes massive differences if the SWA's are terminated correctly?
[ See Murdoch's earlier post ]

Are the SWA cables terminated properly at both ends?

Or look at it from a different perspective..
If you had been asked to design this installation.. what cable size(s) and/or protective devices would your calculations return for the lengths required? Are the existing sizes too small by your calculations?

(And/Or maybe reducing the current ratings of some strategic protective devices to gain a higher max permissible Zs could help?)

And if your 16mm was carrying a full 32A you could go to 77m of cable before exceeding the 6.9volt drop for lights (3%)
Or go to around 128m if only need to meet 11.5voltdrop (5%) for "other" circuits!
(2.8x32x77)/1000 = 6.89v and (2.8x32x128)/1000 = 11.46v

Before doing anything else I would probably want go back and do some comprehensive dead continuity tests across the lengths of ALL conductors to double check you don't have any bad joints or damaged conductors or both, somewhere between supply and the furthest point..?

Then decide what max loads, protective devices, earthing arrangements etc.. are best for your requirements.

 

[mort2376]

Dead testing is a bit of an issue due to certain units being used during the day, but I totally get your point @SPECIAL LOCATION

The smaller single phase boards are fed via 4mm 3core swa. The zs on those are around double what they should be. Some have rcd as main switch, some don't.

Looking at it I'm guessing there will more than likely be some loose glands. I think it's going to be a 3 way approach to sorting it.

1 replace the sub boards that need replacing to make safe.

2 Add additional earth rods or TT if required after remaking off cables in sub boards if zs does not improve.

3 replace rcds in single phase boards with 100a switches and use rcbos on final circuits.

Following the above I think will work towards getting things correct without going over the top.

I can then deal with the basic stuff like exposed singles to wire in the meters after. I'm fairly sure 90% of those meters could be removed.

Of course I do have the exfarmers wallet to try and prise open throughout this !!!!

 

[loz2754]

If you put rods in are you trying to hit a low resistance value or what appears to be the norm these days just knock a rod in and lash an earth onto it

This!
Adding one or more earth rods to a TN system will have minimal effect on the overall Ze/Zs measurements.
For example, say the Ze is 0.20 ohms, and you add one earth rod and achieve an Ra of 20 ohms on the rod (which is in itself not easy to achieve such a 'low' value), the Ze would be reduced to 0.198 ohms.
Better to find the actual reason(s) for the high readings as already suggested by others (poor terminations etc) and then make a plan.

 

[mort2376]

This is my plan. I'm changing 2 boards first which means the terminations will be remade off.

Then I will retest and decide what happens then. It's booked for 1st of October so I'll let you know what happens.

Also been asked to do some juggling of circuits as the battery storage that's connected only applies to 1 phase so they want their office changing to that phase.

 

[UNG]

This!
Adding one or more earth rods to a TN system will have minimal effect on the overall Ze/Zs measurements.
For example, say the Ze is 0.20 ohms, and you add one earth rod and achieve an Ra of 20 ohms on the rod (which is in itself not easy to achieve such a 'low' value), the Ze would be reduced to 0.198 ohms.

I suppose this is one of those installations that probably went in when electricity useage was quite limited and the regulations and testing were not as onerous as they are today and installation design was not as developed as it is now. Having spent just short of 50 years in the industry I have been trying to think when testing became regular thing for every installation and not just for a PIR, yes the IR (megger) tester was common when using Pyro but not always available and identifying the cores was often done with a battery and a small Friedland underdome bell, proving earth was done with the Drummond test lamp (the brighter the lamp the better the earth), ELCB testers in the late 70's / early 80's could be best described as primative to what we have today. My point is has the OP's site ever really met the earthing resistance levels that are now required and how

The TT rod subject always puzzles me, some see it as nipping down to Screwfix, Toolstation etc and picking up a 4ft rod and knocking into the ground connect to the earth terminal and it is job done with no thought to it's siting some are happy to achieve less than 1666Ω while others it's less than 200Ω when you look at resistance vaues on LPS systems it is required that the ground rods / mats must be below 10Ω yet when putting a TT rod in there seems to be a lack of understanding on how best to achieve a low resistance. There is obviously the question of cost but then there is the question of safety
I suppose the installation of earth rods is another of those skills which is no longer taught but it is now used more often than it ever was a number of decades ago

Better to find the actual reason(s) for the high readings as already suggested by others (poor terminations etc) and then make a plan.

There are a lot of electricians who seem to have a fear of the DNO or lack sufficient understanding of the electricity network beyond the service head and the cable that disappears into the ground I have seen it many times where more effort was put into putting a rod in the ground than the simpler option of calling the DNO to investigate the loss of an earth although I have had occasions were the attending DNO engineer has told me it was a TT supply despite the sheath of the PILC sticking through the floor giving very good TNS earth readings on one job there was clear evidence of a metalclad service head being replaced at some point and the earth to the sheath not being reinstated because the metal service head was clamped to the PILC and the customers earth was to the metalclad service head I ended up having to escalate that one with the DNO as the engineer left site adamant it was a TT supply so I requested that they provided suitable insulation on the cable sheath or put an earth lead onto the sheath, one of the office based engineers I spoke to agreed with me and said other properties on the street were down as TNS supplies and the earth was reinstated the following day