EBME Forums Forums Management Medical Equipment Management Yet Another EST Thread

I notice that Frank has produced a nice article about Electrical Safety Testing (with emphasis on the type of situations sometimes encountered in so-called "developing nations").

See here.

Frank has also added two more pages on related topics:-

1) Earthing System and RCD
2) How to Connect a Mains Plug

I haven't got around to studying them yet, except a quick look at the pictures in the last mentioned. I (and no doubt, Frank as well) would be interested to hear opinions on that one (especially the merit of tinning the ends of wires; and the choice of tool [side-cutters] used, etc.). And what about the Hint at the end!

Tinning the wires for mains leads is quite wrong! As a professional you should only use a crimp (ferrule) on the stranded wires for mains connections. Even putting the wires in without ferrules is safer than tinning!

When you tin the solder is soft and will migrate over time making the connection insecure.

As for the note at the end he seems to have no idea about the TN supply system! Clearly there is a need for correct polarity to allow the switches and protective devices to operate correctly and safely.

I would advise he takes these pages down before someone uses it as a defence when something goes wrong!

I must admit I was a bit surprised to see a clearly incorrectly wired UK-style mains plug at bottom right of the "Mains Plug" article.

Hopefully Frank looks in on here from time to time!

Having now had a chance to look through those articles, there are a couple of points I would take issue with, but we need to bear in mind Frank's target audience - what he refers to as "low-income countries"! I'm not sure why he cites IEC-60335, though (as that is the Standard for domestic appliances). However, the leakage current limits given farther down in the "Electrical Safety Testing" article appear to be correct per IEC-60601 et al.

He has to keep it simple (which is fine by me); notice, for example, that he uses the "simplified" (1979) "leakage current test device" at section 3.

On the matter of "tinning the wires" - this is something I also would never do - mainly on the grounds that it means extra work (only joking folks)! It would also mean carting your soldering gear around, instead of just your screwdriver and side-cutters (wire strippers preferred).

However, I seem to remember coming across plenty of cables appearing like that "straight from the factory"; but I always assumed they were tinned simply to protect the bare ends during transit - ready to be trimmed back (cut off) when fitting the plug.

I also seem to recall coming across mains plugs in service with wires broken at the point where the tinning ended - and (as Mike has already hinted at) loosening of the screws at tinned ends, where the screws have "dug in" to the solder (if you know what I mean).

By the way, I would always cut off crimped ferrules at bare-ended mains cables too. I have never really seen the point of them (apart from being just something else to work loose). For one thing, the individual "wire lengths" protruding from the main sheathing always seemed wrong (and thereby difficult to cram into the plug).

All in all, I feel that it's a definite No when it comes to tinning those wire ends; I would always "cut back", and re-terminate using bare wires when fitting (or changing) a mains plug.

Lastly, as Frank has made mention of the UK-style mains plug, I hope that the next article in the series is all about "Fuses"! Although, of course, we need to remember that Frank was most likely schooled in the "European" style of thinking when it comes to electricity supplies - the infamous "Schuko" plug (non-fused, of course), and all the rest.

I have been taught by many different Colleges, Trainers and Courses how to terminate a mains plug. The only one way that I believe is the correct way is the way that I was taught as a Post Office Engineer now alas BT.

1. Do not tin the wires for ac current connections, they will work loose, do not ask me how, something to do with frequency and change of current flow. My own decades of checking connections is that when tinned they do become looses over time.

2. You should always use wire strippers and not side cutters for obvious reasons.

3. The wires should be cut to length leaving the earth wire long enough so that if any wires are pulled out the earth wire is the last to be removed.

4. Ensure cord grip is firmly tightened.

MikeX the system must be TN-S, this is the configuration for a single phase power supply

Hi Guys new to this, but thought I would just throw in my 3d worth.

Cables on appliances are often look as though they have been tinned but quite often the cable is what is called top coated which is done duning the manufacture of the cable, reasoning beind this is that in some lower cost devices internally they either solder the wires directly to the PCB or they use a method commonly called IDC in the connector industry (Insulation displacemnt connector) this is used a lot in Telecoms industry. infact your std RJ45 Network connector uses this priciple.

In Mike's bit about the solder wil migrate over time this usually happens when the ends are been tinned and the solder will wick back up the cable. I totaly agree when pitting into screw terminals this is bad practice as it tends to make the connection very brittle and could lead to broken wires.

I am not sure what the point you are making but a TN-S supply can also be 3-phase. There are many versions of the TN supply system, common ones being TN-S, TN-C and TN-C-S, which is why I just used the term TN to indicate the importance of supply polarity, as the Neutral will be connected to earth (unlike a TT or IT supply system).

Take a look at https://en.wikipedia.org/wiki/Earthing_system for more information.

Just to be clear; it is impossible to assure a mains supply power connection, which is secured by a single screw acting on multi-stranded wire, will provide a constant low impedance during the life of the equipment without the use of a crimp or ferrule.

Omitting such an assured connection means the electrical equipment would fail to meet safety requirements and the duty of care required by legislation. (For example, supply continuity may be critical to the safe operation of medical equipment.)

Take any appliance (medical or non-medical) fitted with a rewireable mains plug straight out of the manufactures box and open the plug. You will always find the stranded wires are fitted with a crimp. If 100% of every electrical appliance manufacturer have made an assessment and come to the same conclusion then why should professionals such as those using this site think differently?

Good point; well made ... but I doubt that we would come across many rewireable mains plugs these days.

However, when a new plug has to be fitted (for whatever reason), how are we supposed to proceed ... ?

Carefully!

As we are generally talking about medical products then we should be making no modifications. Clearly cutting off a moulded plug and replacing it with a rewireable one would be considered a modification. So if you were going down this path you had better be sure you can stand up in court and say your "modification" is as safe as the original.

In any case fitting a ferrule crimp to the multi-stranded wires is going to be an absolute certainty!