A tricky one!!

[ample]

Hi all,
This is not strictly a diy question, but it might get a few people thinking. A job we are doing at the moment is a transformer for a row of industrial units. On top of the cutout is sat a metal CT chamber, with a meter fixed to it. From here goes the consumers tails into an RCD main switch (It's a TT system). Now the problem arises with the fact that the regs say any equipment located upstream of the RCD ( tails, CT chamber etc) have to be class 2. Now the problem is the metal CT cabinet, which we install but will ultimately be adopted by the DNO. The question is bond the CT chamber or not. I know what I would do, but quite a few people I've spoke to have chased their tail with the solution. Would be interested for any input on this odd but increasingly common problem. Oh and assume the metal cabinet is not going to be changed. Thanks

 

[Lectrician]

The DNO will adopt it and will connect it to their earthing system - they would never leave it floating.

They will also provide an earthing terminal, and you can use, or you can leave it.

Sorry, don't really understand exactly what you are getting at? Why would you bond the CT chamber?? Your earthing system and install plays no part in that after the DNO adopt it.

This should all have been sorted when designing the install.

 

[RF Lighting]

I would think if the CT chamber is going to become property of the DNO then the installation will need to comply with the electricity supply regulations, not BS7671.

I don't know what the requirements of the electricity supply regulations are for this sort of setup, but I'm sure the DNO you will effectively be subbing for will.

 

[ample]

The DNO will adopt it and will connect it to their earthing system - they would never leave it floating.

They will also provide an earthing terminal, and you can use, or you can leave it.

Sorry, don't really understand exactly what you are getting at? Why would you bond the CT chamber?? Your earthing system and install plays no part in that after the DNO adopt it.

This should all have been sorted when designing the install.

Its a TT system the DNO doesn't provide an earth terminal. This is the problem, its in a bit of a no-mans land. The DNO will not provide an earth for the chamber. OK, my thoughts on this are that the chamber should comply with class 2 and should not be earthed. The only reason we are getting involved is because we have a duty to flag up problems like this under the EAWR. It just seems anyone who gets asked the question comes up with a different answer. If the CT chamber was made of GRP like most DNO's use, the problem wouldnt exist. Now the tails themselves aren't a problem as long as they enter the RCD enclosure through a non-metallic entry such as paxoline etc. Also the DNO aren't always as up to speed on things as much as people think. Just assuming they know what they are doing, in our experience, leads to issues like this one.

 

[Spark123]

I'm struggling to understand this, your company is installing a supply transformer (i.e. 3.3k - 415v?) and in the units you are using TT? Why not TN-S from the transformer?

 

[Pensdown]

When I first read the post they were my thoughts. Most new industrial unit are steel framed so if they are in a row (as stated) and I assume connected together as normal the last thing I would install is a TT installation.

And as you say, if there is a new transformer on site

But I'm sure ample will tell us more

 

[ample]

I've created a bit of confusion here, and made an arse of explaining it. I'll give you the background on why a TT. A recent occurence for Central Networks was they attended a fault at an industrial unit. The unit was the same as our situation above. ie. 1 steel framed building with several seperately metered units within, but with a PME supply. A neutral had been lost from one of the units so concequently the supply to that unit tried to find a neutral path via one of the other supplies which was smaller in size. While looking at this, they also did some calculations and discovered some circulating currents due to different usage of each unit. The calculations for these are complex, and I'll not even pretend to know exactly what they were. Anyhow, the point is that in situations like this, Central Networks and some other, not all, DNO's have decided the best way forward is by using a TT system. I know it seems a step back, but they have done their homework and this will become an increasingly common sight in the future. Also TNS is not an option as CN only offer TNCS or TT. Talking to a friend of mine who works for CN, he hasn't installed a TNS for 20 years. One of the reasons being the corrosion of the supply cable sheath and affecting the earth. I'm sort of glad this has caused some confusion, as I was struggling to come up with answer myself!? Also thanks to everyone who is chipping in on this one.

 

[Lectrician]

When they offer TN-S these days, they dont use a cable sheath earth.

All industrial units we work on, including the new ones are TN-s from a local tranny.

The cable used for distribution of a PME supply is usually a 3 core waveform, which has the waveform copper concentrics used at the neutral (or CNE - combined neutral earth).

If they supply TN-S they use a 4 core waver form, the fourth core is used as the neutral and the waveform concentric as an earth.

On larger supplies they will use 3 core waveform, but run four length into the building, and use each run for the 3 phases and neutral. They simple put all the three cores into a single lug. The alu cables are tri-formed anyway, so when stripped push together into a perfectly formed circle. With this arangement the waveform is again used as the earth, and usually a bare earth is also run in the trench to the building.

Single phase supplies will be fed with a split concentric.

TN-s is only usually un-available when all other supplies from the same transformer are not TN-s (don't ask me why).

My DNO - WPD - will usually always offer TN-s if the tranny has been installed for units, or if the tranny is soley for a few sites etc.

I actually find industrial units and larger sites are ALWAYS TN-S with PME being used on smaller supplies etc.





Also, your saying that you are having a SINGLE supply installed, and I assume private metering?? What with having a single set of tails after the CT's and a single meter and RCD.

Why then do you want to TT it??

 

[ricicle]

Just out of interest where are you ample?

 

[ample]

LOL, I think you want to re-read the original post, as WE are the ones installing the DNO transformer and cabling and not the consumer installation so i understand all the HV/LV cable types. We effectively work for the DNO in a third party capacity. But they will not permit a TNS installation in this circumstance. As I said, it varies from different DNO's. As far as the original question goes, any ideas?

I'm from Glawster Ricicle

 

[ample]

Oh, and I forgot to say, we recently did a job in the SE for EDS and they wouldnt entertain not using a TNCS for this sort of set-up, so it illustrates that rules vary from region to region?!

 

[Lectrician]

If you are installing for a DNO, you will be working to their spec, and any queries should be addressed to them surely??

You previously talked of TN-S not being used due to cable sheaths deteriorating.....I merely listed the cable types to illustrate that even a PME supply has a'sheath'. Surely this is even more dangerous?

Also, I dont fully understand the install? One incommer?? Whos is the RCD - Not the DNO's surely??

We have done some work for DNO's, mainly the contestable parts of new services and service upgrades / installations. They always still the spec the work, and then check it's to spec before adopting it. Is this not the case with you?