What type and size of cable for a long underground run

[comclusive]

Hi there,

I have a customer (I'm in IT, not an electrician, and am just getting information at this stage before appointing a contractor early next year) who is planning on having fibre and a switch in a couple of fields for events. We will be providing an IP65 cabinet with mains power, a fibre patch panel and a single 48 port network switch. The power requirement is just for one switch and the fans in the cabinet, and will only be powered for the duration of event prep, during the event and on take-down.

We'll be using a cabinet with 19 inch rails for the fibre patch and switch, and a separate locked section for the mains ingress, which event organisers would not have access to. The switch socket will be in this locked section.

The runs will be in a trench and could be as long as 600m. The source is a large (huge) house with a big distribution board.

My question is, for a low power requirement, but with the length of run, what size cable would be best, and do we need to consider inspection points and/or joints? There will be up to 4 of these trenches to different areas of the site, but they may only use one at a time depending on the size of the event.

Also, is it bad to go direct from point A to point B if it involves crossing a field which may have LGVs crossing for event setup and take-down, should the trench go around the perimeter?

Thanks very much

Mike

 

[comclusive]

Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards). If the house is PME supply and you are looking at outdoor stuff you might even want to make this a TT arrangement.

Using transformers for lower drop as @James suggests is possible, but I would suggest unnecessary complexity if you don't really need it.

Cable works out at about £1/m plus VAT.:

Superlec 6942X 2CORE 2.5MM SWA BS5467 STEEL WIRE ARMOURED CABLE - HARMONISED CORES

Please note: If you require advice on which cable to use, how and where to install it, it is recommended that you will need to contact the relevant qualified tradesperson for the job as the Superlec Direct Team are not qualified electricians.

www.superlecdirect.com

You can either use SWA network cable and both can be buried directly in the ground but I would strongly advice using some twinwall duct for both power and fibre and you can use any outdoor style fibre that is strong enough to be pulled through. Mains SWA is safe enough beside even normal copper network cable as it has an earthed screen in the form of the armour. This is the sort of thing (but shop around, this was just my first result that was what I was trying to illustrate) so around £1.60 per meter plus VAT:

Flexi Underground Ducting CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

Flexi Underground Ducting CCTV Fibre Optic - 63mm (O.D.) x 50mtr Green Coil

www.drainagepipe.co.uk

Typically you would want an access chamber around every 200m to allow cable pulling and potentially to allow a section to be repaired. It also allows you to have one run out and then split at the box to serve other end points.

BT/Openreach typically use this system for pavements or footpaths (not on a road where heavy concrete or cast iron cover and a concrete or brick box is then needed for the loading):

Stakkabox Quad Box Chambers

We stock the Stakkabox BT quad box Chamber Access System approved by British Telecom. Order online today, free delivery.

www.drainagepipe.co.uk

However, you can see that a complete box (say 4 * stacked sections to get 60cm depth, and lid) you are looking at around £600 per box.

So for 600m run you are looking at around 4 grand for duct, 3 boxs, and SWA cable delivering 1A power (before you add fibre and network switch). However, if fibre is needed out there then most of that cost is going to apply anyway.

Thanks for the detailed analysis. much appreciated.

 

[comclusive]

If the cable or duct is buried deep enough (say 0.4-0.5m below surface) then it is not important. However, if the field is ever used for agriculture then you really need around 1-1.2m depth.

If this was a permanent system then you might want it routed around the edge of fields to allow for future changes in land use (e.g. adding building) without having to change this, however, that might not be an issue for the owner here.

It's parkland so no agri involved, but always best to plan for the unexpected!

 

[pc1966]

Realistically if you are looking at just a switch and maybe a small defrost heater then 1A / 230V is enough and you could use 2.5mm 2-core SWA with an RCD for supply disconnection (as it would fail on the typical min 6A OCPD for domestic style boards).

Looking at the 2-core SWA cable, for 1.5mm the armour is lower R than the conductor!

But for 2.5mm considered by myself the R1 = 7.41 mOhm/m and R2 = 8.8 mOhm/m, so R1+R2 totals around 9.8 for 600m, say Zs approx 11 ohms allowing for TN-S supply.

That would meet disconnection on a 3A FCU as Table 41.4 has Zs for a 3A BS 1362 fuse as 22 ohms. It still needs RCD protection, but it is always nicer to know you can safely disconnect on the OCPD!

 

[pc1966]

That would meet disconnection on a 3A FCU as Table 41.4 has Zs for a 3A BS 1362 fuse as 22 ohms. It still needs RCD protection, but it is always nicer to know you can safely disconnect on the OCPD!

That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.

While they oly give 3A and 13A in that table, you can also get smaller fuses (1A & 2A) but for typical network switch with switch-mode PSU you run the risk of switch-on surge weakening or taking out too small a fuse.

 

[loz2754]

That was 5s as a sub-main. Probably it would be considered a final circuit where it is Table 41.2 for 0.4s disconnection and then a max Zs of 15.6 ohms for 3A fuse, so still OK.

While they oly give 3A and 13A in that table, you can also get smaller fuses (1A & 2A) but for typical network switch with switch-mode PSU you run the risk of switch-on surge weakening or taking out too small a fuse.

@pc1966 Are you purposefully not taking volt drop into account? If so, may I ask please what your reasoning is behind this?

 

[pc1966]

@pc1966 Are you purposefully not taking volt drop into account? If so, may I ask please what your reasoning is behind this?

The voltage drop is taken in to account for the planned 1A max load.

 

[pc1966]

The voltage drop should be met for the planned use of a circuit, if that is a general purpose one where anything up to the max OCPD rating can be used, then you would size the VD to the OCPD rating. For a fixed load you can size it for the load.

Here the load is small and known, or at least my assumption is it is a network switch which is typically 20W to 150W depending on PoE support, and maybe a very small heater against condensation, like the 40W or 60W things you see for greenhouses and cold frames to prevent frost.

So the OCPD is there only for fault protection, not for overload and not to place any specific limit on the usage. So a 3A fuse is going to meet disconnection on OCPD under fault conditions without relying on any RCD action (and clearly also for overload on 2.5mm!)

 

[pc1966]

I would also add that generally speaking, if you meet 5% VD for the OCPD you will normally also meet disconnection times as the PFC would be 20 times the OCPD (ignoring Ze) hence even a D-curve MCB will trip in the 'instant' region.

For high current circuits you may not meet Zs as Ze is too high and/or the supply impedance itself is a factor in limiting the PFC.

But if you are sizing VD for a much lower current than the OCPD you run a risk of faults not clearing adequately. In many cases the circuit will have an RCD so faults to earth are going to clear by that route, but it leaves L-N faults that may not disconnect at all! The regs don't require a time for L-N faults to clear, only that there is no fire...

That is not something I am happy about in general, so while it is not a requirement, I would still always plan that a L-N fault clears in under 5s and see what means can be used to achieve it. Sometimes the old fuse is your best friend!

 

[loz2754]

The voltage drop is taken in to account for the planned 1A max load.

I see. Thanks for that.
Just did my own calculation on a max design current of 1 amp, with 2.5mm² 2 core swa.
Comes out at 4.97%. Cutting it close, but as has been said, most electronic equipment is designed to operate within a wide voltage range anyway.

Having said that, if going to the trouble and expense of digging trenches and installing ducting, it might be more sensible to design for a higher design current to allow for other equipment to be used.

 

[pc1966]

Having said that, if going to the trouble and expense of digging trenches and installing ducting, it might be more sensible to design for a higher design current to allow for other equipment to be used.

It is certainly something to discuss with the customer to see if they do expect anything else to be added, or if it really is only that.

With duct you have the option to pull in another cable later if that changes, but it is a pain to do so and better to size properly up front.

@James suggestion of step-up and down transformers would be another route, but they have extra losses consuming power even when no load, and it complicates matters as well for anyone who ever comes along later. It might be an option to get ~4 times the power for around £400 to hardware, but you might be better to put that in to running 4mm or whatever to begin with!

 

[SJD]

I haven't noticed any mention of the nature of the events at this site, but can't help wondering what will be powering all the equipment to be connected to the 48 port network switch. I guess that is maybe not the site owners concern, but it would seem a little odd to provide the network connections but no power?

 

[pc1966]

it would seem a little odd to provide the network connections but no power?

Yes, it is indeed very odd.

But if they are looking at powering much out there it is looking to be VERY expensive in cable, or looking at something a bit out of the ordinary like 3-phase maybe going 400/690V on the cable and back down again which ain't going to be cheap either.

If it is for stuff like web cameras then having a PoE switch is really the way to go, and then you are typically in the 150W-ish range, but depending on switch size and total load you might be looking at 500W so around 2A.

 

[littlespark]

Plan your trenches carefully.

If it’s for events, then marquees? I’ve seen a 2ft long anchoring pin burst a 400v 3ph armoured cable

 

[pc1966]

That is a good reason to look at going close to 1m deep even if not agricultural. You can hire digging machines for just this sort of job:

Tracked Trenchers (For Hire)

From soft and sandy to hard as a rock, these tracked trenchers are designed to perform - digging 100mm trenches up to 1.0m deep quickly and

hsstoolshop.co.uk

 

[pc1966]

Out of curiosity I took 2.5mm = 1A on VD exactly and put in advertised costs for cable from Superlec site (ex-VAT) and came up with currents for various voltage options:

CSA \ Volts​	230​	400​	480​	630​	800​	£/m​	For 600m​
2.5​	1​	3.0​	4.4​	7.5​	12.1​	£0.95​	£570.00​
4​	1.6​	4.8​	7.0​	12.0​	19.4​	£1.25​	£750.00​
6​	2.4​	7.3​	10.5​	18.0​	29.0​	£1.80​	£1,080.00​
10​	4​	12.1​	17.4​	30.0​	48.4​	£2.42​	£1,452.00​
16​	6.4​	19.4​	27.9​	48.0​	77.4​	£3.73​	£2,238.00​
25​	10​	30.2​	43.6​	75.0​	121.0​	£5.75​	£3,450.00​

Here I was thinking of @James suggestion and considered:

• 230V is standard single phase (230-0V)
• 400V assumes N is common and L stepped up/down (autotransformers at both ends) 400-0V
• 480V assumes L is mains and other 'N' is opposite phase L (could be isolator uses as autotransformer) so 230-0-230V
• 630V assumes L is direct and 'N' is opposite stepped up to 400V so 230-0-400V
• 800V assumes 230V to 400-0-400V transformers at both ends

No losses or transformer drop considered here, but if you needed something like 16A for powering other equipment remotely it is looking like a cost-effective way either 4mm & 800V, 6mm & 630V, or 10mm & 480V, etc.

However, I don't have any transformer costs to hand, other than a 230V 500VA isolation transformer is around the £150 mark.