Earthing inside a submain panel from main swa incomer

[Guest 004]

OK, going to keep this short.

Supply is 150mm 4 core SWA, with NO supplymentary earth ( calcs done by others, )

QUESTION IS. Do you NEED to run an earth ( is it a regulation) from banjo on inside to earthing terminal inside board. Common sense and a simple ohms continuity test (0.01 ohms) says no. Just want people's thoughts on this.

 

[Darkwood]

Is the gland fitted into a gland plate?
Is the banjo fitted with heavy duty serrated washers to ensure good contact through any paint or the paint has been removed fully from the contact area?
Is the enclosure a 1 piece enclosure or segmented panelling which is screwed together?

If in doubt, tail it out!

 

[davesparks]

Did common sense and simple continuity testing get a value of 0.01 ohms at a test current roughly equivalent to the PFC at the panel? A value of 0.01 measured with a multimeter with a sub-1A test current could be through a tiny spot of good contact which will pretty much melt instantly under the available fault current (which is presumably significant on that size supply)

In short, unless you have ensured a good high current connection to the earth bar by other mans then yes a suitably sized flylead is a damned good idea if not essential.

 

[Leesparkykent]

Reg 543.2.7...where the protective conductor is formed by metal conduit, trunking or ducting or the metal sheath and/or armour of a cable, the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure.

 

[Guest 004]

Did common sense and simple continuity testing get a value of 0.01 ohms at a test current roughly equivalent to the PFC at the panel? A value of 0.01 measured with a multimeter with a sub-1A test current could be through a tiny spot of good contact which will pretty much melt instantly under the available fault current (which is presumably significant on that size supply)

In short, unless you have ensured a good high current connection to the earth bar by other mans then yes a suitably sized flylead is a damned good idea if not essential.

The safety devices will open mate, the sheath will carry any fault currents that are generated, and the protective devices are 80kA. Don't want to go that route really, just want people's thoughts in the real world.

OK, so I m okay with with a 'fly lead' from banjo, so now what size?

 

[Leesparkykent]

The safety devices will open mate, the sheath will carry any fault currents that are generated, and the protective devices are 80kA. Don't want to go that route really, just want people's thoughts in the real world.

OK, so I m okay with with a 'fly lead' from banjo, so now what size?

Adiabatic equation and table 54.7

 

[Guest 004]

Is the gland fitted into a gland plate?
Is the banjo fitted with heavy duty serrated washers to ensure good contact through any paint or the paint has been removed fully from the contact area?
Is the enclosure a 1 piece enclosure or segmented panelling which is screwed together?

If in doubt, tail it out!

Good answer like it.
No gland plate, no serated washers, 1 piece enclosure.

 

[Guest 004]

Adiabatic equation.

OK, that will equate to about a 6mm

Didn't want to go the abiabatic route, because an earth was not being taken back to source, as the the calcs state no supplymentary.

 

[Leesparkykent]

OK, that will equate to about a 6mm

Didn't want to go the abiabatic route, because an earth was not being taken back to source, as the the calcs state no supplymentary.

What's the PFC and what's the over current protective device?

 

[Guest 004]

No supply into the building yet, but the HV chamber is next door, so we are looking at Zs values of 0.01 by calculation, as we are top of the Tx

 

[Leesparkykent]

No supply into the building yet, but the HV chamber is next door, so we are looking at Zs values of 0.01 by calculation, as we are top of the Tx

Ok so you have calculated the Zs of the distribution circuit based on your assumed Ze of 0.01?

 

[Guest 004]

What's the PFC and what's the over current protective device?

The MCCB is Siemens, the frame and type has escaped me for the moment, think it's a BGG on a 'N' curve.

 

[Guest 004]

I repeat the HV chamber is next door, so from thr Tx the run is about 6 metres fed by 630mm parallel supply, so we can safely say that the impedance will be on 0.01. Again I repeat all this has been calculated and all protective devices have been designed to take fault current of 0.01 x root 3.

 

[Darkwood]

If and when you test it, you may have to introduce a fixed resistive component as the TX is so close that the resolution of your testers won't be able to measure it thus your reading will fluxuate wildly

 

[Guest 004]

Believe you me, I will be testing it.

 

[Leesparkykent]

I repeat the HV chamber is next door, so from thr Tx the run is about 6 metres fed by 630mm parallel supply, so we can safely say that the impedance will be on 0.01. Again I repeat all this has been calculated and all protective devices have been designed to take fault current of 0.01 x root 3.

Why haven't they specified the size of the protective conductor?

 

[davesparks]

The safety devices will open mate, the sheath will carry any fault currents that are generated, and the protective devices are 80kA. Don't want to go that route really, just want people's thoughts in the real world.

OK, so I m okay with with a 'fly lead' from banjo, so now what size?

Not your mate!

The sheath won't carry any fault current at all if it is SWA as the sheath is made of PVC, the armour will carry the fault current.
I was not saying anything about the cable's ability to handle the fault current, but that of the connection between gland and enclosure. The connection to the connection to the gland will have to carry that exact same fault current, if it is only making contact on a tiny area like the rough edge of the hole then it may well break down when that fault current flows before the protection operates.

Personally I prefer to use earthing nuts (yes they do make them that big) or else fit a brass gland plate with holes tapped to take the glands of the the SWAs terminated to the board.

 

[davesparks]

OK, that will equate to about a 6mm

Didn't want to go the abiabatic route, because an earth was not being taken back to source, as the the calcs state no supplymentary.

What do you mean you don't want to go the adiabatic route? How did you calculate that the armour is adequate without using the adiabatic?
Supplementary what?

 

[Martin Kerr]

I dont think he is doubting you are not going to test,I believe what he is saying ,is the test equipment may max out at possibly 20 KA and you may not get an accurate reading

 

[Guest 004]

Not your mate!

The sheath won't carry any fault current at all if it is SWA as the sheath is made of PVC, the armour will carry the fault current.
I was not saying anything about the cable's ability to handle the fault current, but that of the connection between gland and enclosure. The connection to the connection to the gland will have to carry that exact same fault current, if it is only making contact on a tiny area like the rough edge of the hole then it may well break down when that fault current flows before the protection operates.

Personally I prefer to use earthing nuts (yes they do make them that big) or else fit a brass gland plate with holes tapped to take the glands of the the SWAs terminated to the board.

Oh I stand corrected, sorry I thought the sheath was the armour.

 

[Guest 004]

Already stated that calcs have been done by 'others' and I'm struggling to see why I need to a adiabatic on a fly lead.

 

[davesparks]

 

[davesparks]

Already stated that calcs have been done by 'others' and I'm struggling to see why I need to a adiabatic on a fly lead.

How else are you going to work out what size it needs to be? It is forming part of the cpc of the circuit so should be calculated the same way.
Your other option is to use the selection table which would have you fitting a 95mm copper flylead.

 

[Leesparkykent]

Already stated that calcs have been done by 'others' and I'm struggling to see why I need to a adiabatic on a fly lead.

So what size are you going to install?

 

[davesparks]

Why has the designer of the installation not specified the method of termination and the size of any flylead required? Or why don't you just enquire as to what it should be? Unless you are accepting liability for this part of the design?

 

[Darkwood]

I dont think he is doubting you are not going to test,I believe what he is saying ,is the test equipment may max out at possibly 20 KA and you may not get an accurate reading

The opposite is what I meant, the resolution of most meters will be to 2 decimal places 0.01, because he is so close to the TX the meter will struggle to verify such a low reading as it will need a minimum level of resistance to settle the reading, as the expected reading is probably going to be at the meters threshhold then it will not find its reading correctly and will generate false readings which will confuse anyone taking them. There are a few options to counter this, buy a nice expensive meter with a higher resolution, or introduce a known resistance into the reading. A simple way would be a local radial socket/spur etc where you can measure the cables R1+R2 giving you a known resistance then you can measure the external impedance from that socket minus the circuits impedance this will then bring your meter back to an accurate reading within its resolution. A simple resistor with high accuracy will also do the trick but not many carry these about as a rule.

 

[Skimpy Wan]

Sorry haven't read the whole post as just walked in the door and the Misses is moaning! Lol

Doesn't GN8 cover what SWA armour can be used as a CPC and which need additional bonding (therfore in parallel).

I'm not sure if I'm allowed to post a link to the page I'm referring to, so please let me know if this is permitted - Mods not us minions! Lol

 

[Skimpy Wan]

If not, maybe I'll see if I can transpose it in a document that is legible for people over the next few days?!

 

[davesparks]

Sorry haven't read the whole post as just walked in the door and the Misses is moaning! Lol

Doesn't GN8 cover what SWA armour can be used as a CPC and which need additional bonding (therfore in parallel).

I'm not sure if I'm allowed to post a link to the page I'm referring to, so please let me know if this is permitted - Mods not us minions! Lol

GN8 does effectively cover it, but it is all there in bs7671 too.
The subject of running bonding conductors alongside an SWA is a completely different matter, and it has presumably already been established that the armour meets any bonding requirements of that there are no bonding requirements

 

[Skimpy Wan]

If not, maybe I'll see if I can transpose it in a document that is legible for people over the next few days as I don't want any repocusions!

 

[davesparks]

You can quote the relevant sections of such a document, or post an image of the appropriate section. But you can't post big chunks of the book like whole pages

 

[Skimpy Wan]

Okay. I'll pay reference to the page / document when I get a chance over the weekend.

 

[Guest 004]

This still will not answer my OP.
FYI the supply cable is FP600, so please post the relevant docs.

 

[Leesparkykent]

This still will not answer my OP.
FYI the supply cable is FP600, so please post the relevant docs.

Your OP asks if you need to have a fly lead from the banjo to the earth bar.....you have had the answer several times and even been quoted reg numbers.

 

[Darkwood]

Reg 543.2.7...where the protective conductor is formed by metal conduit, trunking or ducting or the metal sheath and/or armour of a cable, the earthing terminal of each accessory shall be connected by a separate protective conductor to an earthing terminal incorporated in the associated box or other enclosure.

Not too sure this reg' actually covers the OP's situation, the main switchgear and or distribution equipment isn't classed as an accessory like equipment tapped of each circuit would be IMHO, I did think of this reg' myself but decided it wasn't for this situe'.

 

[Leesparkykent]

Not too sure this reg' actually covers the OP's situation, the main switchgear and or distribution equipment isn't classed as an accessory like equipment tapped of each circuit would be IMHO, I did think of this reg' myself but decided it wasn't for this situe'.

How come you wouldn't class it as the same situation?

 

[Darkwood]

How come you wouldn't class it as the same situation?

Just checking the definitions in the BYB and it seems they have either revised the wording or I'm mixing my regs up as I don't just sit under the BS7671 in my job, so I'll stand corrected on that point, this regulation is applicable although I would argue the case that if you can effectively show you have taked the steps to ensure good solid contact and your not going through gland plates or screwed panelling and the earthing bolt/bar is fixed and grounded to the enclosure directly then you can argue by construction the gland doesn't require a tail.
I think the regulation is too broad in its application but like I said 'If in doubt, tail it out'

 

[Skimpy Wan]

I was referring to GN8 Appendix B (page 161)

 

[Simlec]

I was referring to GN8 Appendix B (page 161)

Would you not use table B2 on the next page? or are you using equipment rated above 70deg?

 

[davesparks]

This still will not answer my OP.
FYI the supply cable is FP600, so please post the relevant docs.

And you didn't think to mention that it's not standard SWA in the OP?

Either way that makes no difference whatsoever to this question.

 

[davesparks]

I was referring to GN8 Appendix B (page 161)

Which is completely useless with regards the question, that only tells you the CSA of the steel armour and compares it to the requirements of table 54.7
It doesn't tell you hat size the cpc needs to be

 

[Guest 004]

Thanks Dave, you have just made me remember why I have not used this forum for a long time.

 

[i=p/u]

Use a 95mm Earth or scratch paint with 2p/screwdriver an use none

 

[Guest 004]

Thank you i=p/u
As the supply is 150, I will probably use a 70mm, but as previously said the ohmic value between the points is 0.01, which suggests to me no 'tail' required. Then you have regulations to contend with. What I was trying to assertain is whether I could use a 35mm as the ohmic value was so low anyway. End of the day whatever I use to 'comply with regs' will just be a cosmetic price of cable, and just a complete waste of time and materials. That said a tail needs (and will) be installed.

 

[davesparks]

How did you arrive at the value of 70mm

Just because the resistance is 0.01 ohms it does not mean it will handle the fault current safely.
You could measure the resistance of a short piece of 1.0mm T&E and get the same result, but it sure as hell won't cope with the fault current

 

[Guest 004]

^ take a day off Dave, and go and read some books.

 

[Leesparkykent]

^ take a day off Dave, and go and read some books.

Why don't you calculate the minimum required csa (not that you have to use the minimum) or use the Table in the BYB? Or you could just take a guess which you seem to be doing.

 

[davesparks]

^ take a day off Dave, and go and read some books.

What's the problem?
I am trying to help you see why getting a low reading is not necessarily proof that you have a suitable path for the fault current.

If I was being unhelpful I'd have pointed out that your use of the term 'ohmic value' is somewhat wrong

 

[Guest 004]

OK guys n girls, first take a look at the table posted below

https://www.dropbox.com/s/820yq4v3l2zj1m8/swa_armour.pdf?dl=0

You can clearly see that a thermoplastic swa (FP600) of 150mm does not require a supplymentary earth.

Now with this FP600 Terminated directly onto the panel, and not through a gland plate, to me this termination is sound, and can handle any fault current. The R1R2 of the cable is 0.01 ohms back to the supply, tested at the gland. This same value is obtained from the earthing terminal inside the panel.

Hope this makes things clear.

 

[davesparks]

OK guys n girls, first take a look at the table posted below

https://www.dropbox.com/s/820yq4v3l2zj1m8/swa_armour.pdf?dl=0

You can clearly see that a thermoplastic swa (FP600) of 150mm does not require a supplymentary earth.

Now with this FP600 Terminated directly onto the panel, and not through a gland plate, to me this termination is sound, and can handle any fault current. The R1R2 of the cable is 0.01 ohms back to the supply, tested at the gland. This same value is obtained from the earthing terminal inside the panel.

Hope this makes things clear.

A table on a page title DIY isn't going to carry much sway with anyone!
Plus that is not for the type of cable that you have, FP600 is a different spec to standard PVC SWA and so you need to get the CSA of the armour from the manufacturer.
But then nobody has questioned the suitability of the armour as a cpc, I think we all know that there are very few situations where an additional cpc is required.

If you are confident that the termination is suitable for the fault current then why did you ask the question in the first place?

If you have a good solid connection from the gland to bare metal then yes it may well be good enough, but a simple low current resistance test won't confirm this.
You are presumably dealing with a significant PFC on a cable that size and so I would have thought a little bit of belt and braces wouldn't hurt to ensure that fault currents will be handled safely.