EBME Forums Forums Maintenance Technical Assistance Micro-Shock data

Good points, Mike; well made.

And that Electropedia link is a good one (that I had not come across before).

Good evening all - I agree too and I wasn't even saying that you should apply medical device standards. What I'm saying is that it is worth understanding the theory of measuring voltages and the individual can make up their own minds. I also said quite clearly that using a good earth resistance meter is sufficient.

So, this thread is about micro shock not being proven and there is no evidence. If this is the case, why bother having a group 2 Medical Location earthing resistance that is lower than group one Medical Location resistances?

May I be the devil's advocate and ask: -

1 Where is the evidence for the IET to justify this different requirement? (Group 1 is 0.7 Ohms and Group 2 is 0.2 Ohms).

2 Why does BS7671 quote a touch voltage for group one Medical Locations and not group 2?

Instead it goes into risk instead - I understand that the argument of a value in G2 is a tricky one but if Group 1 has a value then so should Group 2 - otherwise neither.

The end result of having a lower resistance earth means it better equalises any potential on metalwork which means the touch voltage is less than in a Group 1.

Hope this response is clear and to the point?

For those who don't know - the simple difference (for this thread) between Medical Locations G1 and G2 is that G1 has external applied parts and G2 has internal applied parts such as a catheter.

The difference between a Group 1 and Group 2 medical locations, as defined in BS 7671, is simply down to the threat to life any mains supply failure creates. The reference to applied parts (in group 1) is only their to indicate ME equipment is being used. For group 2 it gives examples where the risk is increased, such as vital operations and intracardiac work. So the definition has nothing to do with any voltage present.

In Group 1 the power supply must be restored within 15 seconds and in Group 2 it must be restored within 0.5 seconds. In Group 2 it is also required to prevent a first fault to earth (e.g. short between live and earth) from disconnecting the supply to other vital equipment on the same circuit. This is often achieved by using a medical IT system (isolation transformer) or supplying equipment from individual circuits.

BS 7671 regulation 710.411.3.2.5 states that for both Group 1 and Group 2 medical locations the voltage between simultaneously accessible conductive parts should not exceed 25V AC or 60V DC. It is repeated in 710.415.2.2 as it was found many failed to take it into account when calculating the maximum values for the protective conductor resistances and simply assumed the limit of 0.2 ohms was OK. There is no difference in the requirements for the conventional touch voltage in either location.

The reason for the different maximum resistance values between Group 1 and Group 2 is really down to the UK regulations being based on the international standards. The values were a compromise for other countries requirements and, as they are only maximum values and not the ones you need to calculate for a proper design, they were transferred to BS 7671. However, in the next version of BS 7671 (18th Edition) to prevent any confusion the maximum value will be made the same for both Group 1 and Group 2. This will also occur at international level but may take time due to the long standards process.

Hopefully the above clarifies the situation.

As already pointed out this is not the correct definition. Also, to avoid confusion, a catheter is not an applied part unless it is directly connected to ME equipment.

Thank you for this wonderful news about G1 and G2 being the same resistance value and your detailed response for this purpose. What is the new value please?

Will there be a new GN 7 soon after as well? :-)

I need to keep having an excuse to post here - still don't like this Dreamer status

While we're on the subject, will the IET be changing the definition of applied part?

Thinking about it, if Group 1 and 2 are the same resistance value there won't be a need to have such a complex differentiation - is this right?

Happy Days - I'm no longer a dreamer