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?