Recently, I’ve noticed an increasing trend in the use of self-tapers to secure earth banjos. The other end of the cable has an earth tail from the banjo, secured with a 6mm bolt and nut. Has anyone else observed this practice, and what are your thoughts on its effectiveness or potential implications? I’d be interested in hearing different perspectives or experiences with this setup.
-Andrew
Hi Graham, I recall that many years ago, our NICEIC field engineer at the time explained that the banjo should be placed on the inside of the enclosure if it’s plastic, as it is considered an exposed conductive part. What are your thoughts on this?
If it's earthed, yes, it's an exposed-conductive-part, but BS 7671 does not require exposed-conductive-parts to be insulated except where protection against electric shock is somehow ineffective, e.g. simultaneously accessible exposed-conductive-parts connected to different earthing systems (Reg 411.3.1.1).
I would agree, though, that sometimes inside the enclosure is absolutely necessary ... for example corrosive environment such as places by the sea, swimming pools, places subject to chemical washing such as food processing plant, etc.
Additionally, regarding the self-drilling screws used in the installation, what action would you take? Would you recommend having them replaced to address the concerns about cut hazards and the suitability of the metal thickness for maintaining an effective electrical connection?
Absolutely, but as this is not immediately dangerous, it would be up to the dutyholder. It's not costly to correct, so I think 'not reasonably practicable' defence wouldn't hold for any of these hazards, including lack of electrical continuity possible.
On this note, I would also point out that there is perhaps a reason why self-tapping fixings may be OK in appliances: where:
The above tests are not really possible to carry out on an installation, which is why a more reliable fastening solution is necessary.
I agree with your comments, and forgot about NEC criteria. Looking at picture closely again there looks as if at least tow if not all of the cables have a seperate integral cable conductor sleeved in green/yellow sleeing so a seperate CPC.
Yes, in addition, I pointed out in my reply to AMK why it's acceptable for products to product standards (in some cases), but not installations.
Personally dont like earth tags on the inside as depending on how things are done can equally be as messy and have earthed parts now closer to live parts.
Agreed - it all depends on the arrangement.
Use of serrated washers and a lock nut with earthing grub screw could actually be better and save the earth tag full stop.
Agreed, where the case of the equipment meets the thermal requirements for protective conductors. Otherwise, I really like 'earthing nut' type products, but again there are sometimes cases where there are limits to space inside modern equipment. Some more thought in the design given to practical installation limitations wouldn't go amiss occasionally.
much more workmanlike solutions (e.g. earthing nuts) are now available which design out almost all these potential problems
Yes, definitely ... unless there's a limitation on space at the cable entry in the enclosure, or, as Gareth says, insufficient clearance to live parts to put a tag/crimp on the inside of the enclosure.
Banjos have had their day
Agreed, and I'm sure if they did not come for free in the gland kit, but instead had to be bought separately and perhaps instead there was the correct size of star washer there would in fact be far better earthed SWA out there. -As the other common SWA fault seems to me an inability to tighten the big nuts so that the cable does not spin within the hole in the box.
Quite how a stalk from a stamped brass shim is supposed to improve over a high compression large area circular contact has always been unclear to me..
More generally looking a those screw spikes one word comes to mind. But as I can't say that word here, instead I'll say
'yuk'
I can't point to a reg, but I'd be muttering about youth of today, lack of standards, and depending how I'd discovered it, the unreasonable distance to run to the first aid kit while dripping blood.
I suppose a design that demands a blood sacrifice before you are allowed to work on it also allows the last man in to be traced by DNA sample. but record keeping and installation certs are more established and the approved way to do this.
Short of tipping the screws in Curare or something it would be hard to do a nastier job.
Mike.
I can't point to a reg,
How about 134.1.1 - good workmanship and proper materials....
- Andy.
When you get up to a 32mm SWA gland there's not much thread to fully locate in the nut particularly if you're glanding into an insulated enclosure with brass earthing plate. The glands could do with a longer thread.
I viewed the photo as yet another example of carp work. If I was supervisiong the work the installer would have to remove the tech screws and re-do the termination with a very blunt and shortly worded warning about comptency and good workmanship.
My view is the followin an SWA, unles there is a specific technical reason for doing so, the paint that will be under the earth ring (AKA Banjo) needs to be cleaned off prior to termination. Then the gland back nut done up tight. The earth ring bolted throught with plated washers and a plainted steel bolt (not brass) and a spring washer or self locking nut. If the galnd plate is removable then a supplementatry earth from the gland bolt to the earth bar.
JP
What is the composition of the screw? Is it stainless steel or just ordinary steel or a coated steel? Some screws use
Carbon steel
Stainless Steel
Brass
Aluminimum
Copper
Bronze
Disimilar metals inside an enclosure. Mmmmm might get some corrosion due to an Anode and a Cathode
Other points to consider will be the screw coating, waxed or treated. Eg Spax have YELLOW which they say offer better corrosion resistance. Other people may just simple opt for Galvanised screw.
Screw head type is also important. Pan head or flange head screws should be what is used but I have seen many installs where a counter sunk wood screw is used. These leaves a raised edge for people to catch there skin on and also more importantly for cables to snag on or cut through the conductor insulation.
Other points to consider. How was that fixing applied? By hand with a screw driver, combo drill or the dreaded impact driver? The impact driver can over tighten and distort a screw, it can also do other strange things to a CU/DB due to its vibration. Further issues may present when these screw are done after the internals are in place as when the impact forces the self tapper into the metalwork there are fragments of metal sent everywhere within that enclosure some will enter the switch gear.
As a side note if I saw that install with the screws pertruding like that I would ask for the person to come back to resolve it. ( cut down at the very least). Those screws look very sharp, sharp enough to cut metal you say. YES, Well WTF will it do to someone’s hands. (Other more polite phrases are available)
My personal preference would be nut and bolt with spring washer for the Earthing Ring in other situation the Pirahna Nut (Other Bands are available) would also be a good choice. It has a smooth side and a serrated or roughned side so it scrapes the paint off to get to bare metal to make a good electromechanical connection.
I dont like to see gutterr bolts used through earth rings. In my opinion gutter bolts are for.......?
JP
Disimilar metals inside an enclosure. Mmmmm might get some corrosion due to an Anode and a Cathode
Good point, but you also need an electrolyte. Any road, aren't those banjos brass and the enclosures steel?
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