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

 

[timhoward]

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.

As you have a TNCS supply and are putting in distribution outside to one giant extraneous conductive part I would agree with your gut reaction about TT.
Before going any further with the other questions what is the current supply, as max demand red flags are waving madly in my head?

 

[John-]

Supply is 100A, TP.

As you have a TNCS supply and are putting in distribution outside to one giant extraneous conductive part I would agree with your gut reaction about TT.
Before going any further with the other questions what is the current supply, as max demand red flags are waving madly in my head?

Supply is 100ATP.

 

[pc1966]

The OSG has tables for BS88 fuse including limits for different CPC size (so factoring in adiabatic values), don't have it with me but they go to 100A.

Otherwise look at the back of the BBB and you have your values for 5s for a sub-main (or 0.4 for final) on the fuse tables, once you have that value you compute the max Zs from:

Zs = 0.8 * 0.95 * 230 / Imax

The 0.8 is for cold measured cables, the 0.95 for near-minimum Uo, and Imax is you current to meet disconnection time.

You won't get much selectivity between a fuse and MCB/MCCB, but look in the tables from the manufacturers (e.g. near the end of the Hager commercial catalogue). For Schneider they have an on-line tool for it, but not all MCB/MCCB choices report selectivity with fuse, don't have link to hand unfortunately.

You could put in your own switched-fuse at the cable feed for isolation, etc, it won't be selective with the DNO but at least you can control the fuses, etc.

In terms of fault protecting the motors there are others on here who know much more about it than me, but look up the typical fuses needed and if less than 1/1.6 of your supply fuse it ought to give complete fault selectivity protection. For example, 22kW 3-phase start-delta assisted start is down as 50A here so is totally selective with 100A or 80A supply fuse:

https://www.hager.co.uk/files/download/0/104407_1/0/TRANSFORMERPROTECTIONVALUEV2.PDF

I.e. consider switched-fuses for the supply isolation and fault protection for those if only 2 big loads.

You should have motor starters with them anyway, probably part of their controls, they would be responsible for overload protection. Unless you have a very good idea that folks are at risk from the slim chance of an open PEN fault stay with TN-C-S.

 

[timhoward]

Supply is 100A, TP.

Supply is 100ATP.

Sorry - I misread your original post.

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?

My procerts "Electrical Tools" app has Max Zs tables for 5 second disconnection of BS88 fuses going higher than 63A.
The value's don't in fact vary considerably between fuse type. (If you were going for TT then less of a problem of course.)

My hunch is you want a TPN fused isolator the supply end, run to container, isolate supply earth there, TP CU in container with RCD TPN incomer, then appropriate C or even D type MCB's for the pumps.

I notice @pc1966 has a different view on the earthing, I'm normally a proponent of keeping the supplier earth but metal containers have always scared me a little more for some reason! He's probably right.

 

[pc1966]

Just to add, if the ground is suitable for a couple of 8 foot rods you could add that to help with the slim open PEN risk, if you are very luck and it is wet clay you might get less than 10 ohms Ra!

 

[pc1966]

Trying to type of a train, not doing so well!

 

[loz2754]

This is basic circuit design. I'm not sure why any adequately trained electrician would struggle with this....(excuse the rant, I'm tired).?

However, the question has been asked, so here's my 2 cents worth.

1. Exporting PME. This is not a thing. We don't export PME. We connect circuits to a PME system. Call me a pedant, but I believe correct terminology to be important, as it means we all understand one another if we're speaking the same language.

2. TNCS vs TT. There are arguments for both systems. There's no particular reason to avoid using the existing TNCS, which provides a reliable, stable connection to earth. Main bonding may be required at the container for extraneous conductive parts. If using TNCS, then the supply cable protective conductor would need to be at least equivalent to 10 mm² to satisfy the requirements for main bonding in a TNCS system.
If using TT, then this would not apply, as the main bonding would be connected to the TT earthing terminal.
Whether using TNCS or TT at the container end, the proposed supply cable itself will of course be connected at the supply end to the TNCS system, and should meet the requirements for ADS up to that point.

3. Use of DNO fuses for protecting a distribution circuit. This is not allowed by DNOs if the cable length exceeds (usually) 3m. So a TPN switched fuse would need to be installed after the DNO fuses.

4. Max Zs tables. You're using the table for 0.4s disconnection time. You should be using table 41.4 over the page, for 5s disconnection time for distribution circuits. The table goes up to 200 amps.
The Max Zs for a BS88-2 100 amp fuse is given as 0.42 ohms. This is before temperature correction is applied, but includes Cmin factor of 0.95.
However, you would in fact be using a lower rated fuse (from your new switch fuse), according to whatever your design current is. So, for an 80 amp fuse it's 0.55 ohms, for 63 amp fuse it's 0.78 ohms etc.

I hope this helps ?

 

[John-]

Trying to type of a train, not doing so well!

Thanks, hope you are not driving the train...

My ignorance knows no bounds... I should have checked the guide.

Thanks for the detail.

If I may.
I will be using a core of the armoured @25mm, and 25mm is not listed for obvious reasons, so 16mm is used for calc so if close, i have some wriggle room - but not much considering most of the value is Ze.

Calculation you provided is the max permissible Zs based on the temp, supply and the absolute maximum disconnection time to comply and fuse type - because it cannot be measured while in use (as it is not installed yet) hence why the application of the formula:

.8 s 20 degree delta to tables?
.95 is 5% Volt drop at supply from 230 - non optimal scenario from supplier allowing for fluctuations also?
230 nominal supply voltage to earth
Imax - from tables at 5S (as this is a distribution circuit).
You mentioned BBB? What's that please?
Thera are considerable variations in current between 88 fuses, (580A, 520A) what is the latest version, or should i always just select worse case (lowest Max permissible Zs)?
Any way to tell without breaking the seal?

This means at the highest let through current for 5s (580) on 88 I get max ZS of .301ohms (520A resulted in .33ohms so went for worst case).

This means, i can now calculate the additional resistance of the cable for the length of run of 25mm Copper conductor and then add in R1 and R2. And as long as my R1+R2 calc values + Measured Ze do not go higher than .301 then i can keep it TNCS. If considerably higher, then i need to move to TT. Also check PFC and check the same. If issue with PFC then bigger cable i guess?

1.454mohm/m @25mm for R1 and R2 (will exclude SWA armour CE for now).
Value already at 20DegC so no need to adjust.
therefore:

(1.454 x 50m)/1000 = r1&r2 = 0.0727Ohms
.301 - 0.0727 = max Ze to comply = .2283Ohms

Was thinking of going Switch fuse isoltor. I also have a socket and light to put in the mains room so will have to create a Hager box with junctions and SPD etc. I also need to run in parallel (temporary) old and new pump. Careful not to overload, so only one of the new pumps will be operating.

So:

100A supply fuses > 100A Switched Fuse > Hager DIN connection box to split supply> Isolator below Control Box in Cabin > Control Box internal isolator.

Hager DIN connection box to split supply>DIN connectors feeding SPD and Local Breaker and 16A RCBO for local lighting and power.

Thanks

John

 

[John-]

Just to add, if the ground is suitable for a couple of 8 foot rods you could add that to help with the slim open PEN risk, if you are very luck and it is wet clay you might get less than 10 ohms Ra!

Are you suggesting that i can install the Rods in addition to the connection to TNCS? Wont that just put me at a greater risk of drawing Neutral currents into my system when a PEN fault occurs?

 

[pc1966]

Are you suggesting that i can install the Rods in addition to the connection to TNCS? Wont that just put me at a greater risk of drawing Neutral currents into my system when a PEN fault occurs?

The open PEN current is the reason for the 10mm minimum copper equivalent for extraneous bonds, as a metal building you have that in any case.

From the Prysmian data sheet for SWA the 4-core 25mm has 70mm steel armour, that is a little under 10mm copper equivalent so for retaining TN-C-S your options are either 5 core SWA (so L1-3, N and E) or cheaper to run some 10mm or probably 16mm in parallel in the duct, etc,

The danger from TN-C-S and metalwork is that it becomes dangerously high relative to the true Earth around it and so a shock hazard under open PEN conditions (I suspect that is a low risk in a isolated area). Adding earth rods helps in two ways:

• Firstly helping "pull down" the neutral/CPC potential as it provides a path for the neutral current to go (possibly reducing the risk of over-voltage damage to thing as well)
• Secondly by "pulling up" the potential of the local Earth close to the CPC/metalwork potential

I think somewhere 20 ohms or below for earth rod impedance in mentioned, look for the section for swimming pools or agricultural special areas.

The BBB = big blue book, a non-obvious reference to the 18th edition wiring regs!

 

[John-]

BBB i shoudl have got that... Doh!

Just putting the design together for this, and am using BS88 bolted fuses. The Current Characteristics scale at this level (compared to a 6A lighting circuit) is pretty scary... It will supply 200A for 1000S before rupturing! Pretty worrying when you think you could use a 70Degree 25mm cable on this. But i suppose as long as Iz is greater than In then all is good. But still...

 

[pc1966]

For fuses typically your disconnection times are met for about 5*In which is scary if you spend too much time thinking about it. But the time/current curves of fuses/MCCB/MCB are all based on the typical overload survival of cables. You just don't want to do it too often!

 

[John-]

Okay, it has been a while since i have had to do these calcs, so am a bit rusty - please be gentle if there are glaring errors, i am trying to do the right thing. I also think that i am being over conservative re the size of the cable. It is only buried for 10% of the length (where it enters and exists buildings) and moves from being a 25mm to a 35mm as a result of ref method D...

I still need to measure Ze, and i think i missed to adjust R1+R2 from tabulated to temp compensated.

Also as i am used to using certain cable types, i never really appreciated how much that the higher Temp (90deg) cables offer (v.s. 70Deg) such a significant increase in CSA - stands to reason i guess. Just need to specify the correct BS when I order the cable...

Attached are the details of the electrical part of the project i have put together, and the calcs i have performed based on the info i have obtained. I have Used the BBB but i don't do these types of designs every day.

Incidentally i will be installing RCDs on the outputs of the speed controller to the pumps, as the Pumps are outside on metal and wooden frames sat on gravel and concreate. They will be interlocked with the fault monition so if an RCD trips, the controller will also be shut down. Resetting an RCD on full load could result in some pretty fireworks...

I would be grateful for any comments please.

Thank you.

John

 

[pc1966]

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.

Estimated 10A three phase for ancillary might be high, that is 6.9kW, but not really much to argue over.

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.

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

From our guesstimated Ze 0f 0.23 ohm we can never meet < 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.

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!

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.

 

[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