Earthed through gland plate screws only

[Phoenix]

Why, everywhere I go do I see boards with a bunch of 4core SWAs glanded into the top gland plate, either with no banjo fitted, or if banjo is fitted no flylead fitted, so the earth for the circuit is relying on the sheet metal screws holding the plate in place:|

Either my inspection standards are too high , there are a lot of rough *******s around, or folk arn't thinking things through fully!

 

[SLIPSHOD & SLAPDASH]

At last, someone else that knows why the banjo is used :Applaud

 

[Riggy]

Why, everywhere I go do I see boards with a bunch of 4core SWAs glanded into the top gland plate, either with no banjo fitted, or if banjo is fitted no flylead fitted, so the earth for the circuit is relying on the sheet metal screws holding the plate in place:|Either my inspection standards are too high , there are a lot of rough *******s around, or folk arn't thinking things through fully!

I agree entirely.

I can almost understand the "no banjo fitted" situation, I assume someone has at least made a judgement (albeit a wrong one in my opinion), but fitting a banjo and not connecting it to anything I really can't fathom!

 

[Evans Electric]

Phoenix , are these older installations , the banjos were often never fitted until the 15 th edition days.

What I find ,of late , is the glands and locknuts are not very tight. Had one last week ,no earth to a print engine, found the cable had been pulled out of the gland which was just finger tight at both ends.

Also came across a dist. bd with aload of SWA's in the top plate , all the banjos on the inside !!!!

 

[wozz]

Out of interest do you banjo MI aswel then?

 

[Sidewinder]

Also came across a dist. bd with aload of SWA's in the top plate , all the banjos on the inside !!!!

And the problem with this is?

I will always fit banjo's & earth wires internal to enclosure where possible.

Earth fault current will flow through the earth tail, into the banjo, through the banjo to the surface interface with the lock nut through thread into the gand body and then down the armour.

IMHO there is less risk of tampering or damage if these items are internal to the enclosure.

You also have a larger surface area of contact between the (admittedly) galvanised steel lock nut and the banjo than you do between the much smaller surface of the gland body nut form.

Admittedly you have dissimilar metals at this joint, brass & galvanised steel, however, there is a big %age of zinc in brass anyway, I don't know how the two materials sit in the electrochemical series, but I'd take my chances and hope that they would be better inside than outside.

Comments?.....

Paul

 

[Sidewinder]

Out of interest do you banjo MI aswel then?

Yes & sometimes even SY depending on application.

Paul

 

[Riggy]

And the problem with this is?I will always fit banjo's & earth wires internal to enclosure where possible.

Earth fault current will flow through the earth tail, into the banjo, through the banjo to the surface interface with the lock nut through thread into the gand body and then down the armour.

IMHO there is less risk of tampering or damage if these items are internal to the enclosure.

You also have a larger surface area of contact between the (admittedly) galvanised steel lock nut and the banjo than you do between the much smaller surface of the gland body nut form.

Admittedly you have dissimilar metals at this joint, brass & galvanised steel, however, there is a big %age of zinc in brass anyway, I don't know how the two materials sit in the electrochemical series, but I'd take my chances and hope that they would be better inside than outside.

Comments?.....

Paul

+1

On the inside wherever possible for me.

 

[Phoenix]

Out of interest do you banjo MI aswel then?

I'd be wanting to see a 16mm (or whatever) earth lugged onto the gland plate if there was MIs made off into it... I would be happy with the same on SWAs, but you may as well just use a banjo on at least one of them to provide the point to lug it to

These can be older installations, but see it on newer too, its not the use of the enclosure as part of the earth path that I have difficultly with, properly done it can result in a very solid connection, but rather its two different bits of metal bolted together with 4 poxy self tappers that I hate...

Dont have much of an issue with banjos on the inside instead of outside, but prefer them on the outside unless there is a good reason to do otherwise

 

[Manator]

And the problem with this is?I will always fit banjo's & earth wires internal to enclosure where possible.

Earth fault current will flow through the earth tail, into the banjo, through the banjo to the surface interface with the lock nut through thread into the gand body and then down the armour.

IMHO there is less risk of tampering or damage if these items are internal to the enclosure.

You also have a larger surface area of contact between the (admittedly) galvanised steel lock nut and the banjo than you do between the much smaller surface of the gland body nut form.

Admittedly you have dissimilar metals at this joint, brass & galvanised steel, however, there is a big %age of zinc in brass anyway, I don't know how the two materials sit in the electrochemical series, but I'd take my chances and hope that they would be better inside than outside.

Comments?.....

Paul

Brass = 0.40 v

Galvanised steel = 1.20 v

Typically there should be not more than 0.15 V difference in the "Anodic Index". For harsh enviroments, and there should not be more than 0.25 V difference in the "Anodic Index". For controlled environments, in which temperature and humidity are controlled, 0.50 V can be tolerated.

Plain carbon and low alloy steels = 0.85 v, so I guess mild steels are better than galvanised ?

 

[Sidewinder]

I'd be wanting to see a 16mm (or whatever) earth lugged onto the gland plate if there was MIs made off into it... I would be happy with the same on SWAs, but you may as well just use a banjo on at least one of them to provide the point to lug it toThese can be older installations, but see it on newer too, its not the use of the enclosure as part of the earth path that I have difficultly with, properly done it can result in a very solid connection, but rather its two different bits of metal bolted together with 4 poxy self tappers that I hate...

Dont have much of an issue with banjos on the inside instead of outside, but prefer them on the outside unless there is a good reason to do otherwise

Phoenix, Why a 16mm for any MI? A 2 core 1.0mm MI should not require a 16mm tail surely!

I will where possible bend banjo's and not even bolt through the enclosure to eliminate the risk of tampering.

Tampering is to me a good enough reason to put everything where possible inside the electrical enclosure.

The increase in resistance of the banjo due to work hardening when bent is negligible, we don't have to consider the work hardening of wiring when installed so why banjo's.

Oh and yes before anyone questions it copper work hardens when cold and when it is bent full stop, so it work hardens when put onto the cable drum, and work hardens again when taken off, and again when bent into place for use. It is just that the degree of work hardening is negligible.

One reason NOT to use solids, e.g. T&E on vibrating or moving items, and the same for heavy stranded 6491x for the same application.

Fine stranded wires do not suffer the same fate as the work hardening phenomenon is related to the dimensions of the material.

Also, work hardening of copper increases its resistance thus this will increase the Joule heating affect and the R1+R2 & Zs values.

These increases are normally immeasurable with the sorts of instruments that are commonly used for testing electrical installations, but they are present.

Paul

 

[wozz]

That was sort of my point pheonix, i hear people complaining about no banjos on SWA but no one ever mentions MI, i dont think ive ever seen MI made off with a banjo. But as you say its mainly in older installations where older substantial enclosures are used as opposed to new enclosures that as you say have four tiny self tappers are used.

 

[Phoenix]

Phoenix, Why a 16mm for any MI? A 2 core 1.0mm MI should not require a 16mm tail surely!

if you only had 1mm MIs on your gland plate, then I'd be happy enough with a miminal earth flylead to the gland plate... though I cant see the logic in putting a 1mm link to the glandplate, when someone might want to come along and fit some heavier circuits and need to replace the link, it just seems daft to me, how much does the foot or so of 16mm cost in relation to the DB you have just fitted, or the long lengths of MI....

 

[Sidewinder]

if you only had 1mm MIs on your gland plate, then I'd be happy enough with a miminal earth flylead to the gland plate... though I cant see the logic in putting a 1mm link to the glandplate, when someone might want to come along and fit some heavier circuits and need to replace the link, it just seems daft to me, how much does the foot or so of 16mm cost in relation to the DB you have just fitted, or the long lengths of MI....

IMHO you should be banjo'ing individual glands.

Paul

 

[Sidewinder]

Brass = 0.40 vGalvanised steel = 1.20 v

Typically there should be not more than 0.15 V difference in the "Anodic Index". For harsh enviroments, and there should not be more than 0.25 V difference in the "Anodic Index". For controlled environments, in which temperature and humidity are controlled, 0.50 V can be tolerated.

Plain carbon and low alloy steels = 0.85 v, so I guess mild steels are better than galvanised ?

OK, lets roll on this.

Who is happy to connect banjo tails up with galvanised steel roofing or gutter bolts.

Who uses brass bolts/screws, who uses stainless steel?

If you use stainless, what grade to you use, A2, A4, Austenitic, Ferritic, Martensitic?

How do these compare on the "Anodic Index" AKA the Electrochemical series?

Further more who is happy to connect to the banjo's by whatever means with zinc plated copper lugs?

Remembering that these same copper lugs will not only be in contact with the brass banjo at their ring surface, but also with the copper wire at the crimp surface.

What is the electro chemical potential between all of these material combinations?

Do we worry about them?

Should we worry about them?

Does anyone understand the possibility of electrochemical corrosion between dissimilar metals at an electrical connection?

Does anyone care about this?

Should we care about this?

Next...

 

[initforthemoney]

I use roofing bolts.

Plus the fact i have yet to find a poor or corroded connection where one has been used.

I think we could get anal about all of this when there really is no need.

 

[oldtimespark]

check out this holesaw it removes paint from around the gland hole. how many times have you seen the main earth taken to the banjo connection bolt then from the banjo bolt to the main earth bar

 

[kme]

I also use roofing bolts - either 12mm , with a bent banjo if there`s room within the enclosure; or bolted through 50mm if there isn`t.

Do I give any serious thought or concern to the dis-similar metals concerns? Honestly, no - I don`t. If we did, we`d be questioning so many things throughout the day, we`d get sod all done!

My opinion, FWIW

KME

 

[Andy™]

i normally use a 6mm bolt/nylon locknut. much tighter connection than roofing bolt, and more resistant to being tampered with

 

[Sidewinder]

I've just had another thought about this gland plate issue with small self tappers holding it on.

The screws will be around 3mm dia, the gland plate will be tight, thus the self cut thread in the enclosure will be bearing on one face of the screw thread, this will be for the thickness of the enclosure material, this could be one thread of the screw that is one circumference.

The peak of the screw thread may also be in contact with the enclosure by the function of the self tapping screw.

Say model the contact area as a toroid of OD 3mm ID 2.5mm, thus 0.25mm depth of thread.

A toroid of these dimensions would have a surface area of around 2.16mm sq.

If you have say 6 of these you would have a steel to steel contact area for carrying fault current of around 12mm sq.

This is a very simple model admittedly, and there are many variables that have not been considered, but, it is good as a starting point.

go up to 12 screws and you have 25mm sq of steel (ish).

So the contact area of these little screws is not insignificant according to this model.

Remember the manufacturers designers may well have undertaken similar calculations when designing the enclosure and deciding on the number of screws to be fitted.

There is also probability factor with regard to the number of circuits entering fault at any point in time for multiple cable entries, and, the PFC/PSCC at the point of the fault, the very nature of the design will limit this anyway.

As long as the circuit makes Zs then you could be fine.

Also these little screws could be galvanised so you will have galvanised steel in contact with bare steel, so another electrochemical issue.

Please guys we need to be careful of NOT over analysing these situations.

One can develop mathematical models to the n'th degree, and they will never meet real world conditions.

Because, once you start looking at engineering problems in this light it always comes down to maths!

You have to make assumptions and simplifications, you'll never have all of the required information to the necessary level of accuracy, so you may just as well say to heck with it and just do the job!

Be careful of how much thought you put into these things or as KME says, you'll end up getting nowt done!

Please keep such things in mind and adopt best practise, yes use the banjo, yes fit a fly lead, suitably sized, remember not to compromise the IP rating of the enclosure by doing this though. Also, clean the paint off around the clamping area as appropriate, though not too much in case you bring accelerated corrosion of the housing into play.

Just let common sense prevail!

Please?

Paul