Can we use the armour of an SWA as the CPC???

[Silly Sausage]

Unless you are installing big boy cables where the armour isn't sufficient, why waste money on an extra core for the earth when you've got a perfectly good enough one in the armour, assuming one buries it correctly (no potential sheath damage)

 

[adiewadie247]

I agree. Using the armour is the way to go but my point related more to the suggestion of attaching a separate CPC on the outside which in my humble opinion is a little pointless. Why have SWA if you going to compromise on the protection of the CPC?

 

[somersetsparks]

I agree. Using the armour is the way to go but my point related more to the suggestion of attaching a separate CPC on the outside which in my humble opinion is a little pointless. Why have SWA if you going to compromise on the protection of the CPC?

The armouring prevents the live conductors from getting damaged etc. even if the cable was crushed it would just short to earth, hence no shock risk. Therefore the earth even if it was squashed wouldn't shock anyone.
Even if you repeatedly drove over a 25mm single cable, you would probably only damage the insulation, not the copper.

 

[Barry Rathbone]

If you even thinking of possible damage to armouring on a SWA cable install a 3 Core for 1 Phase installs and
a 5 core for 3PH installs... Life is more important than money ....Labour costs etc etc on installing seperate earthing. IHMO

 

[Engineer54]

Here is an updated table that i have been using for several years now on SWA compliance, for using the armour as your CPC

Figures in brackets, ... DO NOT COMPLY.



Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	15​	16​	17​
2.5​	5.7​	17​	19​	20​
4​	9.0​	21​	23​	35 ​
6​	13.6​	24​	36​	40​
10​	22.6​	41​	44​	49​
16​	36.1​	46​	50​	72​
25​	36.1​	60​	66​	76​
35​	36.1​	66​	74​	84​
50​	56.4​	74​	84​	122​
70​	79.0​	84​	119​	138​
95​	107.2​	122​	138​	160​
120​	135.3​	(131) ​	150​	220​
150​	169.2​	(144) ​	211​	240​
185​	208.6​	(201) ​	230​	265​
240​	270.6​	(225) ​	(260)​	299​
300​	338.3​	(250) ​	(289)​	(333) ​
400​	403.9​	(279) ​	(319)​	467 ​

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	16​	17​	18​
2.5​	5.7​	17​	19​	20​
4​	9.0​	19​	21​	23​
6​	13.6 ​	22​	23​	36​
10​	22.6​	26​	39​	43​
16​	36.1​	41​	44​	49​
25​	36.1​	42​	62​	70​
35​	36.1​	62​	70​	80​
50​	56.4​	68​	78​	90​
70​	79.0​	80​	90​	131​
95​	107.2​	113​	128​	147​
120​	135.3​	(125) ​	141​	206​
150​	169.2​	(138) ​	201​	230​
185​	208.6​	(191) ​	220​	255​
240​	270.6​	(215) ​	(250)​	289​
300​	338.3​	(235) ​	(269)​	(319) ​
400​	451.0​	(265) ​	(304)​	452 ​

 

[aprillove1719]

In addition to that:
Steel wire armoured cable, commonly abbreviated as SWA, is a hard-wearing power cable designed for the supply of mains electricity. It is one of a number of armoured electrical cables – which include 11kV Cable and 33kV Cable – and is found in underground systems, power networks and cable ducting. Steel wire armour is only used on multicore versions of the cable. A multicore cable, as the name suggests, is one where there are a number of different cores. When SWA cable has only one core, aluminium wire armour (AWA) is used instead of steel wire.

 

[malcolmsanford]

^^^^^^^^^^^^^^^^^^^^ Wouldn't it have been easier to just post a link to

Steel wire armoured cable - Wikipedia, the free encyclopedia

 

[johnboy6083]

Here is an updated table that i have been using for several years now on SWA compliance, for using the armour as your CPC

Figures in brackets, ... DO NOT COMPLY.



Table for 70[SUP]0[/SUP]C Thermoplastic PVC SWA cables.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	15​	16​	17​
2.5​	5.7​	17​	19​	20​
4​	9.0​	21​	23​	35 ​
6​	13.6​	24​	36​	40​
10​	22.6​	41​	44​	49​
16​	36.1​	46​	50​	72​
25​	36.1​	60​	66​	76​
35​	36.1​	66​	74​	84​
50​	56.4​	74​	84​	122​
70​	79.0​	84​	119​	138​
95​	107.2​	122​	138​	160​
120​	135.3​	(131) ​	150​	220​
150​	169.2​	(144) ​	211​	240​
185​	208.6​	(201) ​	230​	265​
240​	270.6​	(225) ​	(260)​	299​
300​	338.3​	(250) ​	(289)​	(333) ​
400​	403.9​	(279) ​	(319)​	467 ​

Table for 90[SUP]0[/SUP]C Thermosetting SWA cables operating at 70[SUP]0[/SUP]C.

Conductor CSA​	Minimum CSA of SWA to meet 54G​	CSA of armour 2 core​	CSA of armour 3 core​	CSA of armour 4 core​
1.5​	3.4​	16​	17​	18​
2.5​	5.7​	17​	19​	20​
4​	9.0​	19​	21​	23​
6​	13.6 ​	22​	23​	36​
10​	22.6​	26​	39​	43​
16​	36.1​	41​	44​	49​
25​	36.1​	42​	62​	70​
35​	36.1​	62​	70​	80​
50​	56.4​	68​	78​	90​
70​	79.0​	80​	90​	131​
95​	107.2​	113​	128​	147​
120​	135.3​	(125) ​	141​	206​
150​	169.2​	(138) ​	201​	230​
185​	208.6​	(191) ​	220​	255​
240​	270.6​	(215) ​	(250)​	289​
300​	338.3​	(235) ​	(269)​	(319) ​
400​	451.0​	(265) ​	(304)​	452 ​

can i just add, that they do not comply with table 54G, but may comply by calculation, using the adiabatic. I seem to remember that the only size that doesnt comply with the adiabatic is 400mm2 2c. This will need you to verify this though, dont take my word for it.

 

[Tonkatoy27]

I have a job coming up in the school hols to supply 2 temporary portacabin type classrooms which will be set up as technology workshops untill xmas. 60amp 60898 3 phase protective device. It's a 150m run which means that due to volt drop I'll have to use 50mm SWA. Now I'd like to use 5core and include an integral cpc but theres about £1200 difference between 4 and 5 core so thats out. I could run a seperate earth if I specially ordered 150m of 25mm which would be better value but is it necassary? From the previously listed tables it's not but as it's a school I'd like to have 'belt and braces'
So my question is, who would run a seperate cpc and who wouldn't?

It'll probably be nicked by pikeys in 10 minutes anyway, they had 100m x 6m of lead off a roof at the same school last year...

 

[malcolmsanford]

Can I ask Tonka first off why the 60amp load, will that be for heating these cabins?

I'm assuming that your taking the VD from the In rather than the Ib ?

Why the 5 core is it a 3phase supply?

Are you going to be classing these as a temporary structure and therefore under section 717. I would think they should be and therefore if you have a PME system you need to make sure you can export it as per reg 717.411.4, which should be ok, just want to make you aware of it.

Personally that long a run I would be looking very closely at the design load, to try and use a smallish cable as possible, especially as it is temporary. I certainly would not be looking at running a separate CPC for it . Is there no design criteria for this by the customer or their representative

 

[Engineer54]

can i just add, that they do not comply with table 54G, but may comply by calculation, using the adiabatic. I seem to remember that the only size that doesnt comply with the adiabatic is 400mm2 2c. This will need you to verify this though, dont take my word for it.

Using the table i re-posted above (post 155) (originally posted by Spinlondon) and i can assure you, ...you won't go far wrong. There is no need for any further calculation to the above complying cables...

 

[Tonkatoy27]

I've been dropped in at the deep end really. Story is: 2x tech workshops burnt out due to unknown cause a couple of months ago, whilst the workshops are refurbished the classes need to go ahead and they've decided to have 2 portacabin type classrooms delivered to a hard standing about 100 meters away as the crow flies. They can't tell me the exact load they will be using, only that each cabin will have 2x 3phase machine circuits. (lathes or pillar drill I expect) a couple of pc work stations, sockets, lights and more than likely heating as they will be up until at least xmas. The sockets will be used by classes using basic tools inc a rack of heat guns, vac pac machines etc. Quite a load if 5 or 6 kids all have heat guns on the go.
I'm going to use the 60amp MCB originally used to supply one of the burnt out classrooms which originally had 5-8 3phase machines in. (con unit well away from fire thankfully)
In used as they can't tell me at the moment exactly what Ib is and I suspect it will creep up and up as they find more machines to runs and it gets colder.

We do all the maintenance and smaller works at the school but installing an IT suite or installing a new 16amp welder socket is a far cry from this.

Thinking some more, it may be more cost effective to come from the existing 60amp mcb (can't change it for a different MCB as its about a 50 years old and no longer availible) to a standalone 32amp switch fuse, then use a much smaller cable? I'd rather do that as I can't believe 150m of 50mm 4core will be very easy to work with
Thanks for your help.

 

[malcolmsanford]

So are you going to give each cabin 30amps of 3 phase supply ( You said your main breaker was 60amp I assume 3 phase 60amp)

So if the cable works out at 50mm and is the most expensive part of the install, then why don't you fit 2 x 32amp 3 phase breakers under the existing 60amp one, and run 2 X 3 phase 32amp supplies out, you most likely would get away with 16mm and possibly 10mm if you had a better idea of your loads

 

[Tonkatoy27]

So are you going to give each cabin 30amps of 3 phase supply ( You said your main breaker was 60amp I assume 3 phase 60amp)

So if the cable works out at 50mm and is the most expensive part of the install, then why don't you fit 2 x 32amp 3 phase breakers under the existing 60amp one, and run 2 X 3 phase 32amp supplies out, you most likely would get away with 16mm and possibly 10mm if you had a better idea of your loads

Ok, that sounds like a winner, I'll get my head back in the books and work out sizes based on 2 32amp instead of 1 60amp. It'll also have the advantage of the rennovation sparkies having the option to supply the 2x32a board from a different place if they need to rip out the existing con unit as part of their work.

I'm going to take a guess at the proposed load per cabin.
2x 3phase lathes or pillar drills
6x 5' flurescents
1x pc workstation
various low powered hand tools. ie, soldering irons.
5x heat guns that may concievably be used at the same time.
2x 2kw convectors when it gets colder.
1x 13amp plastic former

Cable run is through ceiling void to outside, no applicable grouping, thermals or insulation. Clipped direct on wall, 3m duct under steps, clipped direct to fence, 1m duct under gateway, grouped with 32amp single phase swa along fence to cabins.
What size(s) would you go for?

 

[malcolmsanford]

Well mate you have a 3 phase supply so you put the 2 heaters on a phase each so there is 8/9 amps there you have a couple of smallish 3 phase lathes/ drilling machines so what would you say 9/12 amps spread over your 3 phases that make 3/4 per phase so your worse case will be 15amps per phase.

The rest of the load will go on the other phase lights, smallish hand tools etc, former so you may have a load of 15/17 amps per phase.

You could get away with 10mm especially if your being generous and the run is less that the 150 you think it is.

 

[Tonkatoy27]

Well mate you have a 3 phase supply so you put the 2 heaters on a phase each so there is 8/9 amps there you have a couple of smallish 3 phase lathes/ drilling machines so what would you say 9/12 amps spread over your 3 phases that make 3/4 per phase so your worse case will be 15amps per phase.

The cabins are coming pre installed, I'm going to check with the supplier today but you can bet there is only one ring main. If 5 kids are all using heat guns at the time then thats 30 amps pulled through one phase staraight away. :0(
I'll get hold of the tech department and ask them for a list of machines that will be used at the same time to try and get an accurate Ib as unless I'm missing something, I'm still struggling to get my Voltdrop acceptable using 10 or even 16mm over 150m
Thanks for the help.e

The rest of the load will go on the other phase lights, smallish hand tools etc, former so you may have a load of 15/17 amps per phase.

You could get away with 10mm especially if your being generous and the run is less that the 150 you think it is.

The cabins are coming pre installed so I'll bet they only have 1 ring main. If 5 kids are using heat guns then that 30amps through one phase straight away. I'll call the cabin company today and find out.
I'll also get hold of technology and ask what will be used at the same time as if I calculate with 10 or even 16mm over 150m with 32a Ib I'm still struggling to get an acceptable vd. (unless I'm missing something)
Again, thanks for your help

 

[Tonkatoy27]

And the prat of the week award goes to......me. I was indeed missing something.

I was presuming my 3% vd for lighting was 3% of 230v as it was single phase equipment. With a bit of digging I am now wiser. I'm allowed 12v drop not 6.9v

'hangs head in shame'

 

[netblindpaul]

Tonkatoy,Don't forget that Technology classrooms must comply the CoP BS4163.This require further control considerations for the machinery.

 

[Skittles]

Well here is the table to tell you.

That chart is incorrect or at best misleading, for example it appears to state that the swa of a 1.5mm 2core swa cable is equivalent to a copper Cu conductor of 6.85mm ?

 

[johnboy6083]

That chart is incorrect or at best misleading, for example it appears to state that the swa of a 1.5mm 2core swa cable is equivalent to a copper Cu conductor of 6.85mm ?

Bear in mind that the table refers to the heating capacity of the material, and not it's current carrying capacity.So in other words will the material be able to take the fault current for 5 secs without rising to a temperature that would cause adverse effects to the cable.

 

[Skittles]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

 

[stef]

If you take a 10 mm swa two core and take the swa off the cable and bundle it together you'll find that it will make up more than 10 mm so it would be OK to use as cpc. However, as a good code of practice use a separate copper conductor half the size ( in this case 6mm ) additional to the swa as cpc.

 

[Skittles]

If you take a 2 core SWA pvc cable the swa is equivalent to a 4.7mm copper conducter
a 2 core SWA XLPE cable the swa is equivalent to a 3mm copper conducter

Now either we are discussing different matters , or that Chart is seriously flawed ?

 

[Engineer54]

If you take a 10 mm swa two core and take the swa off the cable and bundle it together you'll find that it will make up more than 10 mm so it would be OK to use as cpc. However, as a good code of practice use a separate copper conductor half the size ( in this case 6mm ) additional to the swa as cpc.

Where do you get the idea that running a separate CPC for a more than adequately protected SWA cable is a ''good code of practice'' ?? If you think about it a little more, you may find that it could be just the opposite!!!

 

[Engineer54]

If you take a 2 core SWA pvc cable the swa is equivalent to a 4.7mm copper conducter
a 2 core SWA XLPE cable the swa is equivalent to a 3mm copper conducter

Now either we are discussing different matters , or that Chart is seriously flawed ?

Are you talking about the chart at the beginning of this thread, or the cart i posted (post 155) Which is a far better table to follow...

Anyway we are talking about using the SWA as the CPC, ...NOT using it as a bonding conductor...

 

[stef]

Brass gland and swa in moist atmosphere means contact corrosion ( for some reason it builds up) A crimp lug provides a more secure contact than a brass gland used on a Glasgow isolator. Less mistakes can be made. So, yes, I still think it is a good practise to do so.

 

[Engineer54]

Brass gland and swa in moist atmosphere means contact corrosion ( for some reason it builds up)

In this instance it would be even better practice to replace the shroud with an adhesive lined heat shrink tube!!!

 

[Chr!s]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

Times by 8

 

[Engineer54]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

Both the tables posted in this thread are relating to the armour CSA complying with suitability as a CPC. As i stated previously, the tables do not, and are not intended to be used to size for main bonding purposes. If you require your SWA cable to carry a bonding requirement,t then you'll need to use one of the cores (if 10mm or over) or run a separate bonding cable of the correct size, with the SWA....

In essence, don't confuse CPC's with Bonding, ...they are completely different animals, with completely different compliance requirements...

 

[Guest123]

From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

You need to be careful when considering using the armour as a bonding conductor on a TN-C-S network due to the possibility of overheating of the live conductors under the (very rare admittedly) diverted neutral current situation arising from an open circuited neutral conductor on the supply system.

 

[johnboy6083]

I'm sorry johnboy but I don't get it, it states a copper equivalent Cu, if this isn't meant to be the size of a copper conductor having the same conductance as the swa, then whats the point .
From that chart how would I determine if the swa armour was equivalent or better than a 10mm copper conductor , for the sake of complying with TNCS minimum main bond size ?

the ciopy of the tabvle i have says that : "this table is a guide only, and is for disconnection times upto and including 5 secs. For other times consult the cable manufacturer"

SWA will be able to dissipate heat from fault currents easily because it has more surface area, as the armour is not bunched together.
In the case of the worst type of electrical fault, a long duration, low magnitude fault (like the TNC-s scenario as lenny mentioned), this dissipation will mean that heat is transferred into the inner cores of the cable.

So in other words, the k values which are used to calculate the copper equivelants, and are used in the adiabatic equation are only suitable for short duration faults.

 

[Skittles]

Are you saying that the for swa , for example on 6mm 2core swa cable , the armour has twice the resistance of the 6mm cores, nevertheless because of heat dissipation it is equivalent to a 6mm copper core as stated in the chart ?

 

[johnboy6083]

That's about it, but only for the purposes of Handling fault current for upto 5secs

 

[malcolmsanford]

Times by 8

Wouldn't it be good if more guys bought the GN 8 ...............

 

[Skittles]

Ah so now I see, it is for calculating the armour size suitable for fault currents, but not to be used for calculation of bonding conductors. Looking back through the thread I see I am not alone in being puzzled by this chart.
wouldn't it be good if more guys were perfect

 

[johnboy6083]

Nobody I've ever met is perfect. GN8 is an interesting read though.

 

[Engineer54]

Ah so now I see, it is for calculating the armour size suitable for fault currents, but not to be used for calculation of bonding conductors. Looking back through the thread I see I am not alone in being puzzled by this chart.
wouldn't it be good if more guys were perfect

I don't quite understand why or where you got the idea that these tables were for use in calculating main bonding conductors?? Neither of these tables mentions anything about there use for bonding conductors. The clue is also in the thread title.

The original table posted, can be very confusing to both experienced and novice electricians, expecting far more compliance calculation to many cable sizes than is necessary. The table that Spinlondon first posted fairly early on (around post 34), and later by myself (post 155) and which i have been using for a number of years now, is far more precise for confirming or otherwise, a SWA cables CPC compliance...

 

[johnboy6083]

the way you work out if SWA is appropriate for use as a main bonding conductor is to multiply the mkinimum copper size required by 8.5. The actual steel armour CSA has to be at least that size.

so the minumum size of SWA that can be used for the minimum size of main bonding on TNS (6mm2) is 25mm2. For the 10mm2 minimum copper size of main bonding for TNC-S, the minimum SWA size is 50mm2 4core and 70mm2 3 core onwards. 70mm2 2 core does not comply.

The table posted can be used to find the actual steel armour size, but you cannot use the Copper equivelant CSA from thees tables for main bonding calculation/selection. If you deviode the steel armour CSA by 8.5, it will give you the copper equivelant for main bonding sizes.

As Lenny mentioned, you have to take account of the heating effect that the diverted neutral scenario may produce. In many cases, this may mean that using the SWA as a main bonding conductor is not appropriate.

 

[gonefishing]

Can we use the armour of a SWA as a CPC? you should consider the construction of the cable 1, It's armour protected, bit of a give away, so if it's cut in any way and the armour isn't connected to earth what happens? (that tingling feeling appears).
If the cable route is properly designed and installed correctly and the ........cable is chosen correctly .........underground etc. with an extra core for a CPC and the CPC sizing is correct the jobs good and will last years. I have seen so many SWA installs not earthed in anyway and no CPC whatever and additionally not terminated correctly. I replaced SWA's that were downright dangerously installed after just 4 years, no glands fitted, no armour connected and BS 951 clamps used underground for earth continuity. All this was installed by a well respected company in a millionaires driveway lighting. It's not always "you get what you pay for" And the quick answer is if you have to ask you shouldn't be doing it.... should you?

 

[bryanh]

Running a separate CPC is stupid + it is not practical from a protection of the cpc point of view is it and also it looks bad having a G/Y cable strapped to it in a surface installation,

wouldnt it just be better for the companys who make swa to start including a extra core that brings the swa up to and better then it needs to be, we nearly alway run a 3 core in instead of 2 cores and 5 cores just to make sure that we have a great earth it costs a bit more yes but we think its worth it.

 

[somersetsparks]

Bryanh - I don't really get your post, 3 core and 5 core are available just not necessary most of the time. Using the armour as the earth is cheaper and more practical in most cases

 

[bryanh]

The point was some were saying they dont like using the SWA as a CPC, What I was saying was they could include a CPC within the SWA cable just as they do in T&E etc 2.5+1.5 not a full size CPC then you are covered both ways

 

[somersetsparks]

Ahh I understand, however it would be pointless - The armour provides this function, the only time it would be useful is on cables which dont meet the minimum CSA of armourings. I don't understand the covered both ways bit though - You are covered if you install a proper and continuous compliant earth, in most cases the armour provides this, in fact I don't see how people are happy to trust a bit 1.0 cpc in a screw terminal which has probably chewed it down to 0.5mm on a lighting circuit, but frown upon using the mass of armourings, terminated in proper glands? beats me

 

[johnboy6083]

the only time i wouldnt use Armour as CPC was a) i had lots of 3 core spare, b) it was going to be installed in an environment where if the sheath were damaged, excessive corrosion would cause failure iof the Armour as CPC, C) in a TNC-S scenario where i could see the armour carrying diverted neutral currents, i would assess this ti the best of my knowledge and then decide.

 

[KAS1]

Nothing wrong with using the SWA as the cpc, although most companies I've worked for over the years always use a three core for single phase and four core for motor cables so as to use the spare core as earth together with the armour.

 

[aleksandar]

Yes, you can use SWA armour as a CPC. If you use the adiabatic formula you can check if the armouring is suitable or not.

 

[MDJ]

Of course it can be used and will probably be adequate, have a look at the table engineer put up recently, however If I was doing the job I would run a 3 core rather than 2 core and use both the armoured and the third core together, and banjo the ends as well with fly leads to the earth terminals, I really cannot see the problem with this sytem and providing it is allowed for in the fixed price quotation why wouldn't it be a good idea not to use a 3rd core as well as the sheath?

Mike

 

[Engineer54]

Never a good idea to rely on the armour to make up any deficiencey in the size of a 3rd core CPC. Being as there is a big difference in K values and therefore will not divide equally to carry a fault current to operate a protective device. Where a 3rd core is employed as the CPC, it should be of a sufficient size to provide the Zs required, to trip the protective device and carry the full fault current of that circuit.

 

[MDJ]

The third core would probably be adequate in most circumstances of course, the sheath would be connected to it via a fly lead to the banjo for professional reasons, I can't imagine running in an armoured cable and not connecting it to an earthed point, so having both of them connected together at an earthed point would be normal and not to ensure a correct sized cable, as you said the cpc should sufficient in size, but they are bound to be connected together.

 

[SLSpark]

ARMOUR FOR EARTH OF COURSE THATS WHAT ITS THERE FOR(and underground protection etc) IF TERMINATED TO THE CONTAINMENT (trunking) and that should be earthed anyway then you have just made the armour part of the earthing system common practice to run seperate earth along with armour , if actually seen this done with a 5 core armour which gives a 3rd earth overkill maybe but the earth is the most important. ever heard the saying "ITS JUST AN EARTH" without a earth it shouldn't be connected