EBME Forums Forums Maintenance Technical Assistance Micro-Shock data

Historic documents often refer to the potential for Micro-Shocks, in their effort to justify following certain practices within medical locations, such as the use of isolated power supplies.

However, I and others have been unable to find any statistical evidence of any deaths or injuries from such phenomenon. If anyone knows of any verifiable statistics concerning Micro-Shock please let me know.

Also if you have any data in general pertaining to unintentional electric shocks where medical electrical equipment is being used this would be useful. However, again I am not sure any exists since medical equipment is so well designed and maintained these days. A tribute to all those who frequent this site!

Wikipedia.

But I recommend Dyro* (on this matter and many others) - especially (in this case) Malcolm Ridgway's famous "The Great Debate on Electrical Safety - in Retrospect" (found at Chapter 65).

* 19 Meg pdf

Here are a couple of interesting (and to my mind, still pertinent) extracts from the document (Chapter) cited above:-




Dr. Bruner also pointed out all those years ago that:- "Electrocution occurs when, and only when, an individual becomes the component that closes the circuit in which a lethal current can flow". Fair enough.

To answer your original question, Mike ... if you read the Chapter I have indicated you will see how the famous "1,200 deaths a year" estimate arose, and how it has been massaged over the years by folk pursuing various agendas. Names are given, but I won't repeat them here.

I think the reality is, at this moment in time, there are no recorded incidents of micro-shock or even marco-shock within the medical environment due, in no part, to the robust standards applied to both the medical electrical equipment and the electrical installation coupled with the regular testing performed on both.

Some ,who have used the information from the distant past, have tried to justify additional measures without any data to back them up.

I just came across this and thought I'd reply. Yep - it's me who can be included in the above last paragraph. I was responsible for the first MEIGaN around 1998 so it must be me. MEIGaN as you know was produced at a time when BS7671 was completely out of kilter with Medical and considering I had no support from above with this document - it moved things on somewhat at the time - and good that it is now within BS7671 which is now how it should be. MHRA won't go outside their device remit (not past the mains plug pins these days).

You are right about the lack of data - there has never have been any but having spent time teaching this for the last 6 months, I think it is good practice to be aware of the methods available to check a new installation. You can soon see if the installing electrician has done it right or not. If it looks to be a bad one then it's worth seeing if there's any touch voltage present as well as checking the earth resistance .

I agree entirely that with modern methods and approaches that touch voltages will not be an issue in general but if clinical engineers understand what is needed as a whole, then they will be armed with the skills and knowledge for that one day when there is a bad install - which is becoming more and more of a rarity.

So far, on all of my courses, everybody agrees that being able to understand that the end aim of a good earthing system is to get a low resistance which means no real touch voltages. They all find my explanations helpful: including what region of voltages they should be looking for if they feel they should test for a touch voltage - if it ends up as say 19mV and the resistance is OK, then there's no need to take action but if the resistance is borderline and there is say 35mV present on something in the patient zone, then it is worth looking further into the issue.

Respecting views on this lack of data, I now promote an approach as with radiation protection. The risk is stochastic so the best approach is to minimise the risk. This is done by fitting a radial earth system with cable that is thick enough along with other good practices. However, electricians don't always get it right so as us oldies are getting older, we need to teach the full electrical science so that they can continue the battle to ensure every new room complies. This includes knowing how to make more checks if you think a room does not comply.

*ALARA?

Devil's Avocate, Ian:- what is all this re-engineering aimed at? How many folk have actually been zapped by "micro-shock"?

For what it's worth, my experience of New Hospital Projects (working with architects and so forth) drew me into the "Hospital as a System" approach; here we would look at (design) each room "in the round", as it were. In other words attempt to blend in Best Practice (not to mention Building Codes, Fire Codes, Disability Access - and all the many others) in every case.

Actually, the electrical side of things is just one piece of the puzzle; and it's a bit late once the hospital has been built, the kit installed, and the patients begin piling in through the door!

BTW, I wish we had had MEIGaN in our Reference Library back in those days.

* As Low As Reasonably Achievable

Geoff

You got it!

No re-engineering here. No proof they have and it can't be as stopping the heart of a human or pig doesn't exactly follow modern day ethics (or Kent if you are Chris Eubank). The latest HTM does not mention touch voltages in group 2 areas as it was deemed that MEIGaN was too zealous. I can't argue that an installation if done to BS7671 should be checked but what I'm on about is a back up plan and the locals can judge whether or not to use it.

Your approach is right - HTM06-01 promotes this practice in the guise of the Electrical Safety Group which as you say is just one piece in a massive puzzle. England is quite varied in this - whereas the other UK administrations take a more focussed approach

Successful Projects are generally the result of having a strong Project Manager.

And (unfortunately), the reverse is almost always true as well.

In a word:- Leadership!

Whether in battles, wars, equipment (or hospital) design ... or engineering projects!

To be honest, Ian, my take is that Mike was referring to those "less than scientific" studies carried out (a couple of which I cited farther up the thread) - and the questionable results proclaimed to the world - back in the 1970's.

But let's not forget that it was that sort of scare-mongering that played a large part in creating the whole "biomedical engineering" scene (to which, for better or worse, many of us have given a big chunk of our lives).

Anyway, no doubt Mike will be along to respond, sooner or later.

First I would say that the term "Micro-Shock" is not an IEV defined term and does not appear in any published IEC standard. So we can't look for or measure something that is not actually defined!

Second we need to be careful about making an assumption that a voltage of a few millivolts can have a detrimental effect on the human body. It is the current that is of concern and in order that this can flow it must overcome the barrier of the skin.

The only connection between the electrical installation earth and the patient is via the protective earth conductor. As the only part of an electrical installation that should contact a patient would be by them touching an earthed enclosure or maybe via an earthed part of an examination table top, any voltage would have to first break down the barrier of the skin.

Applying a few hundred millivolts to someones skin can't cause any hazard, otherwise an AA battery would have to come with a safety warning and a PP9 would be considered a lethal weapon!

If you look at IEC/TS 60479-1 - Effects of current on human beings and livestock, you will find much data on the effects of electric shock but in particular notice they only provide data for voltages above 25V. This is why voltages below 25V are considered safe (SELV limit is 50V for standard locations and 25V for medical locations).

We also can't start applying IEC 60601-1 to the electrical installation or we would have to CE mark the whole hospital!

The historic (50 years ago) issues were that most electrical installations in medical locations were poor and many items of electrical equipment used were not certified as medical devices and in many cases were never maintained or operated correctly. The only harm patients are exposed to these days are when medical staff fail to follow proper procedures or use equipment not in accordance with the manufacturers instructions. For example, connecting non-medical equipment to medical equipment, such as when some video equipment (microscope etc.) used in the patient environment is connected to equipment outside the patient environment (monitors and recorders that are not medical). Today, medical equipment meets high safety standards as do electrical installations and both are maintained routinely. Patients have never been more safe from unintentional electric shock.