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Protection of sub-main from DNO
jack2006
10 Posts
Question

Supply is being upgraded to 3-phase 45KVA with quoted PSC of 25KVA and as the DNO equipment will be housed in an extrnal cabinet a PME connection will not be provided so a TT system. To provide protection to our 4 core 25mm XPLE SWA and the DNO's equipment intention was to install a TP&N 63A switchfuse. Problem though is that the switchfuse enclosure is steel so both that and the SWA armour only has the earthrod fitted adjacent to the cabinet for protection. The rest of the existing installation having RCBO's and its own earth rods. Any thoughts on a solution for this?

22 Replies
Zoomup
4231 Posts

Upstream suitable R.C.D. in insulated enclosure.

Z.

jack2006
10 Posts

Sticking point is the 25kA quoted fault current which is unlikly at the moment as the supply before going in our duct underground from pole is overhead for about ½ mile from the DNO substation. Anyway assume by some change of circumstance we see 25kA at their cutout then I can see where you are coming from as the PSC will fall over our 4 core SWA run. I would have sooner put the RCD adjacent to the cutout but cannot achieve the PSC quoted with the RCD's I have found all being 10kA

AJJewsbury
3144 Posts

I very much doubt you could have anything like 25kA for a earth fault current on a TT installation. Firstly 25kA is likely quoted for a L-L-L fault rather than an earth fault - which is much more likely to be quoted at the normal 16kA level, and secondly unless you've massive extraneous-conductive-parts shared with a neighbouring installation that uses the PME facility from the same substation/LV network then the earth fault loop is likely to be limited - in an ideal case by the consumer's electrode with nothing in parallel with it - in which case you'd be looking at fault levels of a few tens of amps, never mind kA.

In any event with large fault currents the HBC fuses should open well before the RCD (especially a time delayed RCD), so the RCD still wouldn't have to break such currents.

   - Andy.

AJJewsbury
3144 Posts

I'm not sure why the supplier's earthing facility can't be used anyway. There are a number of situations where PME isn't recommended or actually banned, but I've not heard of receiving a supply into an external cabinet being one of them.

   - Andy.

jack2006
10 Posts

Yes I would be suprised to see the 25kA and from distant past (35 odd years ago) it used to be the standard value given by a DNO. Only really valid if you were next to the transformer. So the solution in those days was to take the actual PSC value at origin, which was usually around 5kA , use Ottermill breakers which from memory were the only ones quoting a PSC value of 10kA at the time and fitting a Traffolite label stating the design max PSC value.

With regard to the provison of PME the DNO has stated that as their equipment is not located inside a building and instead is housed in a freestanding cabinet then PME connection will not be provided. 

Issues also arose when our industrial building was constructed and as steel portal frame we were not allowed a PME connection as one of 5 in a block.

mapj1
4444 Posts

A 63A 3 phase supply which measures a 25kA PSSC will be a record.  

If you include 3 ‘death or glory’ fuses of say 100A or 80A rating  then downstream of them the let-through energy will be limited to something equivalent to PSSC below 10kA anyway, so the use of more conventional parts will be OK. 

If there is a PME terminal available, use it at least for the case of the first isolator, or it really needs to be in a plastic enclosure.

As a side note, earth fault relays are made for PSSC higher than this, but you will need a stiff drink after you have read the price - there is a sharp increase as you enter the world of the industrial.

Mike.

 

Chris Pearson
3262 Posts

That supply is on a domestic scale so it seems surprising that an external cabinet rules out a TN earth. More likely would be the length from the transformer - difficult to see how you would get Ze below 0.35 ohms. (PSSC won't be sky-high either.)

jack2006
10 Posts
The DNO has an earth connection at the bottom of their distribution pole where we installed our cable duct from, so this will be the point they have created their PME I guess. As its a rural location the only properties being fed from this pole is this one and a neighbours that used to be a farm and they already have a 3-phase connection with PME terminal.

Supply change is planned for a couple of weeks time so I think the best appraoch is to ask for clarification as to why a PME terminal cannot be given even if its just to protect the TP&N switchfuse as the rest of the instalation does not require it.

Since doing these posts I have looked again at the DNO's original spec/offer and they state 3-phase 45kVA supply with PME which in my world equates to 63A and a PME connection. Then on the site plan which arrived weeks later for our trench excavation it shows the poles distribution, earthing etc and its stated as protection by 80A fusing and no PME so think clarification is required.
alanblaby
115 Posts
I'm involved in a similar install, (along with the Consultant, John Peckham). We have been told pretty much the same thing, we need to supply the cabinet, we will be getting a 3 phase supply (only 1 will be used) and will have to supply our own earthing system. The overhead pole is around 6 metres from the property wall. I have already excavated the area, fitted an earth mat, and a base for the cabinet. Just waiting for WPD to come to dig up the street and fit their head. The property is around 70 metres from the pole/wall. I'm too, a little bit worried about what to fit in the cabinet. I've not found a plastic clad switch fuse. I could fit the Wiska type large bushed ones to keep the live conductors well away from the metal work, but, my current thought is to fit the time delayed 300mA RCD directly after the meter in a plastic box. The tails can then go from this to the switch fuse, for the outgoing SWA to the house.
jack2006
10 Posts
Initialy this install will be similar just using one phase with DNO moving the single phase meter into the cabinet. My original thoughts were to use a 300ma time delay RCD to protect our cable and its enclosure which had to be fireproof to meet the DNO's spec so thnk that puts it as steel and not plastic. Not sure how the meter and cutouts work with this though as they are plastic??

So problem with the RCD is still the PSC that DNO has quoted which I think we all now agree to a bit over the top.

Another option I am looking at is the modular DIN rail HRC fuse carriers from Legrand and if a combination of 3 phase switch for isolation, 3 HRC carriers and then the 300ma RCD are offered in a steel enclosure. 

Still going to ask for clarification though as to why a PME terminal cannot be provided as this would be the most straightforward solution.
alanblaby
115 Posts

jack2006: 
Initialy this install will be similar just using one phase with DNO moving the single phase meter into the cabinet. My original thoughts were to use a 300ma time delay RCD to protect our cable and its enclosure which had to be fireproof to meet the DNO's spec so thnk that puts it as steel and not plastic. Not sure how the meter and cutouts work with this though as they are plastic??
 

Fibreglass is what they require. Almost all new small sub stations are in a large GRP enclosure. What they mean by fireproof is they don't want a cheap adaptall plastic box to connect into. (A few) Domestic electricians have form for fitting consumer units that catch fire (partly due to Manufacturers claiming they do not burn, but not then actually meeting the Standards when they were made.)

Search for GRP enclosures and you'll see what I mean, they are built to meet various Fire Ratings. The DNO will probably suggest a minimum size too, I've gone for something like 1000x600x400mm, floor mounted.

jack2006
10 Posts
Sorry, thought you meant the RCD or switchfuse enclosure. Cabinet we have used is lockable IP rated 800 wide x 900 high x 320 deep.
Attached files
fiftyhertz
62 Posts

I’ve used this in the past, you can also request a GRP enclosure for TT systems. 


Alternatively a Micrologic with a Vigi earth leak in a box will do a simialr thing

 

 

jack2006
10 Posts

Protek solution looks good, cost is a bit steep though at just over £1K so will contact DNO today to see if we can a PME just for our equipment/cable in the cabinet

mapj1
4444 Posts

If you get through to someone in engineering rather than ‘have a nice day’,  I'd try asking for a more accurate estimate of loop impedance and PSSC as well - it should be possible to calculate fairly close if they can tell you the cable size, and you can take the distance off the map.  

And do not forget the energy limiting effect of the fuses (After all, the same thing allows 6kA MCBs on a line with an apparent 16kA PSSC with a BS1361 /BS88 style house service cut-out.).

Mike.

Well on one of my construction sites, I have installed a Schnider NSX400N IEC 60947-2 with a bolt on additional adjustable  RCD at the bottom of it - in my case set at 300mA. 

(Construction Site) There must be a 63 Amp version available? 

The supply tails are L+N only and enter the metering equipment first, (I assume in your case - at the supply pole?) then through the top of the main switch in through a  paxolin top (The top plate of the metal enclosure has been chucked out, the paxolin installed in its place).

 

300mA RCD bolted on the bottom of the main switch. 

 

From there your SWA goes off to your site?

 

Another great place to look for these sorts of ideas are Blakely - they do a lot of site temporaries DBs and pre-made DB 400/230/110V commando sockets arrangements. 

 

My go to for pre-made assemblies that might well be exactly what you want……..

AJJewsbury
3144 Posts

With regard to the provison of PME the DNO has stated that as their equipment is not located inside a building and instead is housed in a freestanding cabinet then PME connection will not be provided. 

The DNO aren't assuming this is a temporary builder's supply by any chance?

     - Andy.

jack2006
10 Posts
No the chap came out for a site visit to put together a quote as it is a change from overhead single phase to existing property to an underground 3-phase. Originally their equipment was going to be mounted in an enclosure sunk into the properties wall but the cable run was a pain due to existing workshop for the house being in the way. So it was his suggestion of a freestanding cabinet where their cable enters the properties boundry gave the most flexible solution for future building work that will be being done.
jack2006
10 Posts

Thanks for link. Had a look at the MCCB route but cost is an issue and awaiting a call from DNO to see if anything can be done about a PME connection.

Chris Pearson
3262 Posts

Any idea of the cable size and, therefore, the PSSC?

Zoomup
4231 Posts

Glass reinforced plastic (G.R.P.), as found in caravans and boats,  is normally very flammable. I presume that electrical cabinets made of this have a fire resistant/suppressive  additive.

Z.

jack2006
10 Posts
Yes these cabinets are used for external distribution and simialr ones were used by my company many years ago for streetlighting on private estates.

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