IET logo
 
IET
Decrease font size
Increase font size
Topic Title: 185mm Armoured paralleled
Topic Summary:
Created On: 10 August 2017 01:24 PM
Status: Read Only
Linear : Threading : Single : Branch
1 2 Next Last unread
Search Topic Search Topic
Topic Tools Topic Tools
View similar topics View similar topics
View topic in raw text format. Print this topic.
 10 August 2017 01:24 PM
User is offline View Users Profile Print this message



antdon

Posts: 86
Joined: 07 November 2007

Hi All

Ok... A biggy ...

I am looking at an installation of probably 2 x 185mm armored cables burried 200M long and (wait for it) Paralleled....

I HAVE NOT DONE THE PROPER CALCS YET (so be patient)..

This is to supply 400A to a Miniform panel board.. (200M away)

My questions for anyone having done this before are:-

1. Should I be looking at 5 core cables or 4 cores with separate earths..... ( would the separate earths buried be within regs)?

2.. What problems has anyone experienced in a similar installation?

Any comments would be gratefully recieved...
 10 August 2017 01:54 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16083
Joined: 13 August 2003

As it's buried you'll presumably want an armour that's suitable to act as a c.p.c. - so why not use that for earthing the remote end? i.e. 4-core + armour, nothing separate. (unless of course you have some odd PME bonding requirements that push the required c.s.a. well beyond what's required for the c.p.c.s.)
- Andy.
 10 August 2017 03:11 PM
User is offline View Users Profile Print this message



OMS

Posts: 22423
Joined: 23 March 2004

If it helps, and this kind of arrangement is my day job, the 185mm2 are probably the right size just by looking at the "regs" but if you have something akin to "normal" ground then 120mm2 would suffice (assumed soil resistivity of 1.2 and temperature of 15C)

The armour alone would be fine - however, I would tend towards running a separate 70mm2 single insulated copper earth cable with the parallel pair of 4 cores - put the two cables bang tight together and the single earth fits neatly with that. It helps with EFLI for circuits downstream of the miniform unit

You don't want to be buying 5C x 185mm2

Regards

OMS

Make sure you tell the switchpanel supplier what cable he needs to accept - you don't want to be struggling to get two cables into a switchboard design for a single cable incomer

-------------------------
Let the wind blow you, across a big floor.
 10 August 2017 04:06 PM
User is offline View Users Profile Print this message



antdon

Posts: 86
Joined: 07 November 2007

Many Thanks for the reply...

I wasn't sure whether you could bury a singles withe the armored cables.

4 core sounds a better (cheaper) option.... And I would imagine a much easier install....


Many Thanks
 10 August 2017 04:06 PM
User is offline View Users Profile Print this message



antdon

Posts: 86
Joined: 07 November 2007

Many thanks for the reply....
 10 August 2017 04:46 PM
User is offline View Users Profile Print this message



Jaymack

Posts: 5377
Joined: 07 April 2004

Originally posted by: antdon
I am looking at an installation of probably 2 x 185mm armored cables burried 200M long and (wait for it)

I'd be considering 3 x 120mm², makes for easier labour handling and increases redundancy ...... albeit still limited. But consider the costings.

Regards
 10 August 2017 06:24 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16083
Joined: 13 August 2003

I'd be considering 3 x 120mm², makes for easier labour handling and increases redundancy ...... albeit still limited. But consider the costings.

But might make fault protection more interesting (worst case you'd end up have to install individual s/c protection at both ends of each core) - see 434.4 & appendix 10.

- Andy.
 10 August 2017 09:35 PM
User is offline View Users Profile Print this message



davezawadi

Posts: 3953
Joined: 26 June 2002

I'd listen to OMS Andy. Consider the I2t of a single core under fault conditions. The result is not individual fault protection!
Regards

-------------------------
David
BSc CEng MIET
david@ZawadiSoundAndLighting.co.uk
 11 August 2017 08:01 AM
User is offline View Users Profile Print this message



antdon

Posts: 86
Joined: 07 November 2007

Many many Thanks for all the replies.

Looks like it will probably be 2 x185mm plus the earth (thanks OSM) The ground for most of the run is just soil, but 20-30M will be through an area of sandstone. So, will be trenched and sand filled.....
 11 August 2017 09:34 AM
User is offline View Users Profile Print this message



kenelmh

Posts: 92
Joined: 17 February 2012

Originally posted by: davezawadi

I'd listen to OMS Andy. Consider the I2t of a single core under fault conditions. The result is not individual fault protection!

Regards


I think it could but perhaps not that likely..

We don't know 'Ze' so we don't know the fault capability of the circuit.
For instance, with a fault from a single leg of the parallel circuit to earth, the Zs may be too high to cause ADS in a satisfactory time. hence then, individual protection would be required - right?
 11 August 2017 11:20 AM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16083
Joined: 13 August 2003

For instance, with a fault from a single leg of the parallel circuit to earth, the Zs may be too high to cause ADS in a satisfactory time. hence then, individual protection would be required - right?

That's another possibility. The point I had in mind was that while a "half size" conductor would almost certainly be protected (thermally) from faults by an OPD sized to protect the whole conductor from overload, when you get down to one third, you're getting much closer to 'it might or might not' territory - depending on lots of factors including actual loop impedances and type & setting (and therefore resulting energy let-though) of the OPD. I'm sure we all remember that days of 2.5mm2 with a 1.0mm2 c.p.c. (opposite end of the scale of course, but same underlying principle).

- Andy.
 11 August 2017 11:52 AM
User is offline View Users Profile Print this message



OMS

Posts: 22423
Joined: 23 March 2004

Well - we know the source end protection is 400A - that should give you a good estimate of the source impedance

From there do a few sums ?

Decide exactly what the fault is - e.g. Totally severed core with only one end to earth - intact core to earth - severed cable with the partner intact - both cables severed etc

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 11 August 2017 12:46 PM
User is offline View Users Profile Print this message



AJJewsbury

Posts: 16083
Joined: 13 August 2003

Ah, my bad. From the suggestion of using three 120mm2 in parallel I'd got it into my head that each would be about a third of the required c.s.a for current carrying capacity - now I've got my head on the right way up that's obviously way out - each would comfortably handle half, so I agree, little to worry about. Sorry for the diversion.
- Andy.
 15 August 2017 10:06 AM
User is offline View Users Profile Print this message



antdon

Posts: 86
Joined: 07 November 2007

Many Many thanks for all the replies.
OMS - Thank you very much - Alway nice to get a reply from someone who does it daily
The more I have read, the more confusing it gets... lol
But it now looks like either 2 x 150mm (over egged) OR 2 x 120mm As OMS Suggested seems to be about right..

Found a great online cable calculator at http://myelectrical.com/tools/cable-sizing-calculator
 15 August 2017 05:04 PM
User is offline View Users Profile Print this message



Nedryerson

Posts: 108
Joined: 12 December 2009

Just a thought but sand is a lousy conductor of heat. The cables will be happier in riddled stone-free backfill.

Ned
 15 August 2017 05:23 PM
User is offline View Users Profile Print this message



Nedryerson

Posts: 108
Joined: 12 December 2009

Just noticed the length of the run is 200m !

I get 2 x 240mm sq to comply with the VD of 2.5%

Ned

BTW Do you know the fault level ?
 15 August 2017 07:09 PM
User is offline View Users Profile Print this message



OMS

Posts: 22423
Joined: 23 March 2004

Originally posted by: Nedryerson

Just a thought but sand is a lousy conductor of heat. The cables will be happier in riddled stone-free backfill.

Ned


I hesitate to say that sand back fill would be "worse". Dry sand is a good insulator, sure - but add a bit of ground water and any quartz based material that has minimal air spaces will outperform soil with a variable degree of organic content (and at the same compaction)

The one advantage of sand is that it's easy to predict performance - whereas with "soil" you need to know a bit more about it

Probably why I assumed the thermal resistivity was no better than about 1.2K.m/W

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 16 August 2017 08:10 AM
User is offline View Users Profile Print this message



Nedryerson

Posts: 108
Joined: 12 December 2009

This question gets more interesting by the day !

I just noticed that the cable route will be through Sandstone so the choice of backfill will depend to some extent on the duty cycle. If, for example, the cables are fully loaded 247 (such a wind turbine) then sand would dry out quickly and overheating could become an issue.

My calc is still indicating 2 x 240mm sq because of volt drop and these cables will be awful to terminate into a miniform even with an extension box fitted. (is the Miniform new ? If so, probably wise to go for the 630A incomer with solid links)

Is there not a case here to revisit the actual power requirements of the load ? Often, this turns out to be a fraction of the installed capacity.

For such a long run disconnection times may be difficult to achieve so determination of the source impedance is crucial. How is the upstream switchgear configured? BS88, MCCB?


Ned
 16 August 2017 06:51 PM
User is offline View Users Profile Print this message



OMS

Posts: 22423
Joined: 23 March 2004

Is 2.5% volt drop limit perhaps a bit ambitious in this scenario

I think I'd be minded to think a bit more ESQCR and take a limit on the submain to the miniform panel at about 5% - allowing say 3% at the far end for any further circuits - or even a full 10% from source to load if it kept the miniform feeder size "reasonable"

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 16 August 2017 09:04 PM
User is offline View Users Profile Print this message



Nedryerson

Posts: 108
Joined: 12 December 2009

Agreed.

Using the original 2 x 185mm sq returns 2.62% VD and conductor temperature of 60.39 degC.

Still concerned about disconnection times.

Ned
IET » Wiring and the regulations » 185mm Armoured paralleled

1 2 Next Last unread
Topic Tools Topic Tools
Statistics

New here?


See Also:



FuseTalk Standard Edition v3.2 - © 1999-2017 FuseTalk Inc. All rights reserved.

..