BS88 max ZS table for fuses over 63A TT or not TT

[John-]

Hi

First time with this kind of a job for me.

I have a scenario where i am planning to install about 50m 25mm TPN from a TNCS supply to supply a small metal container housing TPN pump control gear to control two TP 24KW .8 PF pumps (to be fair one is a 15kw, but allowance to swap out one for a bigger one later). Design Current and Volt drop so far state 25mm will be fine.

Because of the distance i would have thought exporting PME would not be a good idea and was going to make it TT as there would be metal railings, the container itself etc. and we are in a very rural area, but...

Speaking with the Pump installers supplying the control gear they do not convert to TT, they always use the earth supplied - but to be fair they are also very close to the supply and have very low ZS.

I have not yet measured the ZS (nor prospective), but i expect ZS will be .2 or above and the supply fuses i am told are 100A, newly installed by DNO. The client took responsibility for this.

First Q, if i am to rely on the suppliers fuses (to protect only the cable between the meter room and the metal cabinet) and install only an isolator at the mains intake to isolate the supply, does anyone have any charts available please that cover Max Earth fault Loop impedance values for the BS88 fuses? Regs book only goes to 63A. BTW the pumps are individually OC protected in the control gear and internal wiring in the control gear to this point is also 25mm.

Also how do i tell what version fuses they are without breaking the seals?

I do not like the idea of leaving the suppliers fuse as the only means of protection for the 50m run of cable into the control gear - it just feels wrong?

I need to confirm but i am pretty sure there is a lockout on the control gear so that both pumps do not start at the same time. So even at 43A (one pump starting) there is a lot of load for about 10 seconds or so TBA.

So if i look at putting in an MCB at the meter end, i need to be sure that they will discriminate (something the pump guys said you will not guarantee even if you had scope to lower the rating of the MCB to 80A) between DNOs and mine and i tend to agree. The pumps, whilst having a slow feature start, can draw a heck of a lot of current on start up, meaning that a C Type MCB maybe the only reliable option, even assuming that the pumps do not start simultaneously. .22 Max ZS for a 100A Ctype, so an MCB may not be an option anyway, may already be too high for the length of riun and Ze. So if cannot discriminate or be selective, what is the point of having additional OC at the meter - that is my thinking. If worst happens and there are issues, i can leave BS88 fuses on site to replace any that go. If they do go though, that's going to be one hell of a fault or lightning strike most likely needing more than just a fuse...

So assuming the maz ZS for the 100A 88 fuses is greater than that measured at the end of the cable supplying the isolator for the control equipment, then TT would not be needed. If the max ZS was lower than the calculated value, and increasing the cable size was not practicable, then I will have to make it a TT and supply a 100mA S type RCD when it comes into the cabin, disconnecting PME and staking it.

Welcome yo ur thoughts please.

Thanks.

John

 

[John-]

Thanks forth detail! Appreciate your time!

Here goes: If you are looking at a 100A supply fuse as overload protection then yes, it probably needs to be 35mm cable in duct. However, you could get away with 25mm for the full running current of both motors and for a volt-drop point of view. Personally I would go with 35mm as then it is met for open 70C cable and so the end accessories are not at risk of working with 90C or thereabouts cable. very good point concerning accessories temp.

Estimated 10A three phase for ancillary might be high, that is 6.9kW, but not really much to argue over. - Agree, may need small heater at some stage and right now the Pump control box is providing a simple SP 20A fuse in the control kit, was planning to split this down to lighting (6A RCBO & 16A RCBO Radial - single socket) - utilisation yea next to nothing but it does nothing to impact cable size .

Looking at the Hager guide for 45kW motor assisted start it has 80A fuse as protection, 22kW individually as 50A fuse, so your 100A fuse is fine even for simultaneous start of both motors.

100A fuse in Table B3 of the OSG has 0.34 ohms as measured (i.e. cold) Zs for meeting 5s and also adiabatic for 16mm CPC so no issues there.

If the supply is TN-C-S and reasonable, say better than 10% drop on full load, then your supply Ze is going to be around 0.23 ohms (23V = 10% at 100A, probably less, but to be measured or inquired) so max R1+R2 for sub-main limit is 0.34 - 0.23 = 0.11 ohms.

Assuming 35mm 5-core cable then R1+R2 as seen from volt drop tables as 1.25 mOhm/m so length limit is 0.11 / 1.25E-3 = 88m and as longer than your 50m planned length is met.

If you are really paranoid about worst-case cable fault you would look at armour resistance and work out the worst place for a fault, etc. Looking at the Prysmian data sheet for 35mm 5-core the armour is 1.6 mOhm/m so the worst location works out at 34.9m from the supply end and R1 there is 0.022 ohm and R2 is 0.028 ohms (armour from source in parallel with remaining armour to end and the 5th CPC core back to source) so R1+R2 worst-case fault location is 0.05 ohms and less than the above limit of 0.11 ohm based on a guestimated Ze and fuse for 5s sub-main. Hmmm. Nice!

Alternatively you could say max Ze = 0.34 (max fuse) - 0.05 (worst case R1+R2) = 0.29 ohm.

At the pump end you are either looking at 63A D-curve MCBs or 50A BS88 fuses for the pumps, based on Hager data sheets (page 102 of commercial one here Hager Brochures, Catalogues & Documentations - https://hager.com/uk/p/catalogues-brochures). From the OSG Table B6 final circuit values for 63A-D we don't have Zs limit nor does the OSG give 50A final circuit fuses (not really domestic or common commercial I guess?). So let us compute the 63A-D MCB from max "instant" trip of 10-20 * In so max = 20 * 63 = 1260A so we can compute Zs limit as:

• Zs = 230 * 0.95 * 0.8 / 1260 = 0.14 ohm

Adjustable overloads are already provided within the controller by the controller supplier - it is a built up panel, with alarm inputs, outputs and such like i need to check the overloads find graphs, ZS etc for them and check the supplied cable tales on the pumps to make sure all is well. Wil be annoyed if not as pumps and controller came form the same supplier - cracking job on the panel... Plan was to protect the Pump control panel supply cable - (35mm / everything) with a single TP 100A switch fuse back at the meter, then install an isolator local to the pump control panel before it dives in. Internal cabling is 25mm Tri rated up to the Overloads.

From our guesstimated Ze 0f 0.23 ohm we can nevermeet < 0.4s for the 63A D-curve MCB, we would need to rely on and RCD to meet disconnection times if socketed. If hard-wired, as probably the case, we can allow 5s as for the sub-main and that is listed in OSG Table B6 at 0.28 ohms for the MCB so we might be able to meet it if the supply Ze is a little lower.
They are fixed, but outside the EP zone, so will be providing RCDs, mounted inside the cabinet after the speed controllers. I need to get RCDs with a jockey on them, so i can also shut down the speed controller when the RCD trips. Mounted inside the controller, so you have to isolate the supply to reset them - otherwise big risk of someone resting them on full start load (after the speed controllers have ramped up) with pumps in stopped sate - no back EMF and a big bang

Further more looking at the Hager tables on fuse-MCB selectivity we have none with a 100A fuse, so this is not a good design as a hard phase-neutral or phase-phase fault could take out the supply fuse(s) with any RCD seeing nothing wrong with such a current path. That is surprising as the 63A C-curve MCB is selective to 4.4kA faults so probably fine at end of cables, but alas not for the D-curve recommended for such a motor!
I cannot see how i can discriminate here. Other than under rating the 88 fuse at 80A (the graphs show they will take it) - even then as you say a hard fault will take the lot out. Honestly i was going to snip one of the supply fuses record exact details of the exact make and keep some on site for that eventuality if it occurs. I have not checked but the Pump guys said that MCBs will be too much trouble and yo wil never get eth ZS at D rating - i still need to look at graphs for them etc though.

50A BS88 fuse from table on Fig 3A3 has 0.4s at 380A so Zs limit is:

• Zs = 230 * 0.95 * 0.8 / 380 = 0.46 ohm

As we already have to meed 0.34 on the sub-main for the supply fuse we could allow a further R1+R2 of 0.12 ohm and meet 0.4s final disconnection. Furthermore a 50A BS88 fuse is totally selective with a similar 100A upstream fuse, so this is a good choice. Just not so convenient as a couple of breakers in a board.

As above, if hard-wired then 5s is permitted and the OSG table B3 has Zs max at 0.79 ohm and adiabatic met for 6mm or more at that time.
Thank you again, i guess you do this day in day out - nice one . I did the C course way back when and almost went on to HNC/D but, we hit a recession and i had to work nights, then left the company not long after any way. Never too late i guess

 

[pc1966]

Keeping "10A" 3-phase allowance for extras is not a bad idea, as you say lights, a socket or two, maybe fixed heater if a lot maintenance in cold winter likely.

Realised I made a mistake in the worst-case armour location as I solved for the highest R2 location, not the highest R1 + R2 location. Actually result for same assumed values (0.625 conductors from VD tables, 1.6 armour, 50m total) is 0.054 at 48.2 meters, so not much far from 0.05:

If there are dual pumps for redundancy reasons you really don't want a fault on one taking out the main supply fuses

Based on what I calculated some 50A switched-fuses might be better as combined isolators and final circuit protection, and then a small CU or DIN board for the distribution of power, extra circuit's RCBOs, and any SPD needed.

You would get some selectivity between, for example, the Hager '160' series 100A MCCB & 63A D-curve MCB, but then you won't have it against the DNO's fuses here (if it is 100A supply) so no better off. I would stick with some TPN switched-fuses for both supply and final pump supplies.

 

[John-]

Hi
Thanks again for the thorough dive into this.

I went to site got bitten to buggery by ants (okay technical they squirt), horseflies and Mosquitos...

Worse case Ze i measured was .27ohms @843A, best case .24ohms. I have taken the worst phase here.

Up until last night, with revised cable length of 60m it was just in with 1.1mohm/m for a 70degree multicore armoured, buried - yes I know 'It' is 98A - what's a couple of amps between friends... To be honest, there is only a short part under ground. So i think i would have slept okay with this.

But...

Customer may be changing the cable direction, more buried and more length - total not yet known.

With 60m it was just in (without taking into account armoured) :-

Max Zs 100A BS88: 0.34Ohms.
Measured Ze worse case: 0.27Ohms.
Max (R1+R2) (0.34 - 0.27): 0.07Ohms.

(r1+r2) value for 35mm 70degree, muticore TP: 1.1mOhm/ m

0.07 /(1.1/1000) = 63m

So it was close at 60m measured. Of course I have not taken into account armoured, so the CE (@1.6mOhm/m) there will help a bit so.

1.6mOhm/m + 1.1mOhm/ m = 1/1.6 + 1/1.1 = 1/ 1.534090909 = 0.65ohms

So my mOhm/m resistance value including armouring and the 35mm earth = .65Ohms - this could then give me a new max length of: 0.07 /(.65/1000) = 107m. But...

As you deftly pointed out yesterday a worse case fault at any point in the cable will result in some energy going back up the cable armouring, and back down the 35mm. I now need to figure this method out, I may cheat and use a spreadsheet to calculate the fixed measurements...
I'll be back!

John

 

[pc1966]

Bottom of this page has cable R1 and armour resistance values:

BS5467 Cable - XLPE Armoured SWA Low Voltage Power Cable

BS5467 Cable | Prysmian REQUEST QUOTE | Thorne & Derrick UK & Exports for best prices, service and fast delivery worldwide.

www.powerandcables.com

Can't find the PDF on line but this is my copy with info from a year or so ago:

 

[pc1966]

Stuffing in the PDF values for 35mm 5-core and 63m I get:
Enter line R1 in mOhm/m 0.524
Enter CPC R2 in mOhm/m 0.524
Enter armour R2 in mOhm/m 1.6
Enter length in meters 63
Maximum fault R1+R2 = 0.059 at 55.6 meters

 

[John-]

I think i got to the same / similar answer:

r1 will always increase as you move away from the supply.
A will also increase as you move away from the supply
B will decrease as you move away from the supply
C will be a constant but is in series with B.
Both B+C are in parallel with A.

Therefore

r1+r2 = r1+ 1/((1/A)+(1/(B+C)))

Bugger, wont let me attach a spreadsheet.

But yea, around 50m i make it

 

[pc1966]

I think i got to the same / similar answer:

r1 will always increase as you move away from the supply.
A will also increase as you move away from the supply
B will decrease as you move away from the supply
C will be a constant but is in series with B.
Both B+C are in parallel with A.

Therefore

r1+r2 = r1+ 1/((1/A)+(1/(B+C)))

Basically that is how I calculate it, but using some python script so I get a pretty graph out of it, etc. Putting in your numbers I get:

Enter line R1 in mOhm/m 0.55
Enter CPC R2 in mOhm/m 0.55
Enter armour R2 in mOhm/m 1.6
Enter length in meters 107
Maximum fault R1+R2 = 0.104 at 96.7 meters

This forum won't allow spreadsheets, no doubt it won't allow python though I could just paste the text to allow others to use it / check it.

 

[pc1966]

Realistically you will struggle to meet disconnection for 107m under those values on the OCPD alone.

You might find it better to look at putting in a fancy adjustable MCCB style of RCD at the feed end set for 1 sec delay and 0.5A or more trip so you might get some selectivity with final circuit 50A BS88 fuses if they can clear a fault in under 1s (as well as no issue with double pole 30mA RCBO or RCD)

You might even get away then with 4-core and parallel 10mm together with the armour as CPC, saving a small bit to help cover the fancy RCD cost!

 

[pc1966]

Another option would be parallel runs of 16mm SWA or similar for each pump if that would allow 50A switch-fuses at supply to disconnect, but location for isolation might require another one closer, etc.

 

[pc1966]

Two 16mm 4-core in parallel have enough SWA to meet TN-C-S 10mm copper CPC equivalent and if putting in values it comes to:

Enter line R1 in mOhm/m 1.15
Enter CPC R2 in mOhm/m 3.1
Enter armour R2 in mOhm/m 3.1
Enter length in meters 107
Maximum fault R1+R2 = 0.289 at 107.0 meters

That along with your supply Ze would be fine for 50A fuse on 5s disconnection (0.27 + 0.29 = 0.56 < 0.79 ohm). You would need yet another SWA for the ancillary stuff to a small CU but if 10-20A then it could be smaller, even single-phase, or maybe just more 16mm.

Cost would need looking in to, but it might actually be easier to wrangle 3 * 16mm 4-core in one duct/route than 35mm 5 core.

 

[John-]

Another option would be parallel runs of 16mm SWA or similar for each pump if that would allow 50A switch-fuses at supply to disconnect, but location for isolation might require another one closer, etc.

Overloads are already provided in the controller.
The run should not be anywhere near 100m...
More worryingly for me right now is that our answers do not match, inclined to think that it is my maths not yours .

I would love to have a look at your code, but right now i need to get my head around what i am doing wrong here...

I found some bracketing errors and sorted those but still not matching. Then i realised that i may not have solved the parallel resistance values correctly for A, B and C?

I am now wondering my assumption that B and C are a simple series circuit in parallel with A is the issue:

This is the formula i used

r1 + (1/((1/(B + C)) + (1/A))) = r1+r2

r1 is in series with A, B and C so no special care needed for r1.

A has a parallel path back with B and C. And B and C are in series, but collectively in parallel with A.

Any ideas please?

This is the spreadsheet with the formulas revealed-: -

Cheers

John

 

[pc1966]

The effective R2 is A in parallel with B+C as you say, can look later at your numbers and another look at my code as well, out at the site so don't have it with me!

 

[John-]

Thank you , that would be great .

I didn't know if it was more complex, along the lines with this example - but it seem you agree with me and therefore it is not: -

If my math is correct i am scuppered anyway, because at just after 40m 5 core 35mm i exceed max ZS just after 40m away form the supply: -

 

[John-]

Spotted an error on the value for R1 for 35mm, i incorrectly stated r1+r2 = 1.1, but is should be 1.048 - not that it made any significant difference.

 

[pc1966]

Checking the numbers I used before getting 0.104 ohm at 96.7m of 107m we have:
R1 = 0.55 * 96.7 / 1000 = 0.053185
A = 1.6 * 96.7 / 1000 = 0.15472
B = 1.6 * (107 - 96.7) / 1000 = 0.01648
C = 0.55 * 107 / 1000 = 0.05885
B+C = 0.07533
R2 = A||(B+C) = 0.050663
R1+R2 = 0.103848 which is 0.104 ohm to 3 digits.