The relevant AU/NZ standard (that is to say AS/NZ 3003) requires the measurement of earth bonding from GPOs to the EP point and voltage potential from any exposed metal to the EPJ in a patient treatment zone designated as "cardiac" (generally a theatre with RCDs or LIOMs). To do this requires the ability to measure very small resistances and voltages sometimes across substantial distances. To do this we generally use a locally made EST device known as an MEM which allows a 4 wire (2 wire constant current and 2 wire sense) system (see the attached). What I'm wondering is what do others use? The Rigels don't do this but do any other makes or is different method is known? The issue with the MEM is simply one of size not of function so I am looking for a more compact device for when I'm on the road.
Thanks all, Dave
Thoughts and information provided on this forum are mine and mine alone and do not necessarily reflect the policy of NSW Health. They may also be complete bollocks!!
If you need to measure the protective earth resistances then any meter designed for the task is suitable as long as it can provide a minimum of 200mA. In the UK we can use a Rigel or similar but as this is overkill. As the task is generally performed by electricians they use a standard electrical installation tester, such as a Metrel MI3125, Megger MFT170 or Fluke 1652c etc.
In the UK we don't to need to measure so called touch voltages (e.g. 10mV, 100mV) as this makes no sense. What you are trying to do is actually measure is leakage currents, so really you should be using a standard IEC 60601-1 human body model, as built into devices such as the Rigel 288. The question is why? Do you expect the electrical installation to be contacting the patient as an applied part? If so you would need to get the entire medical location tested as medical device!
If you just use a high impedance voltmeter to measure voltages between the earth reference and parts in the room you may measure high voltages, such as a automatic tap (faucet), which use a class II power supply. This could be as high as 110V but, if loaded by a 1K human body model, the voltage will collapse and you will get just a small leakage current (less than 250 micro amps).
One joke about the 10 and 100mV levels is a medical IT is allowed 0.5mA leakage so you would measure 500uA from this with a first fault to earth condition, so making a mockery of the lower two levels. Touch leakage and applied part leakage should be only of concern to medical devices and have nothing to do with the electrical installation. Here the proper equipotential bonding provides all the protection you need, when combined with RCDs and Medical IT systems.
If you really need to measure very low resistances with high accuracy then a four wire meter is available from Seaward, which is part of Rigel, called the Cropico DO4002. But what you really need is your standards to be brought into the real world!
But what you really need is your standards to be brought into the real world!
and there are many who would agree Thanks for the links, gents, have not come upon them before so might be worth a look. If any others have thoughts please feel free.
Ta, Dave
Thoughts and information provided on this forum are mine and mine alone and do not necessarily reflect the policy of NSW Health. They may also be complete bollocks!!
The Cropico certainly looks a bit handier ... and is apparently available from RS (and Farnell).
I must admit that, like Mike, I was wondering why you're being called upon to make those measurements, Dave.
OK - because they are called for in the Standard, I guess; I wonder what "they" base all that on?
BTW, I have noticed on other forums* that electricians (as opposed to biomeds) are well provided with a vast range of measuring instruments. Some of those blokes can get very er, anal about the intricacies and minutiae of this and that testing circuit etc.; as well as discussions (arguments) about the relative merits of each method. So perhaps it would be better to leave all this "building services" stuff to them.
* And yes, the Fluke 1652-C gets mentioned (indeed praised) a great deal.
I use the CROPICO earth bonding tester, which will measure down to micro-ohms. This can also be useful when checking switches and contactors. I carry a 10m and a 20m two core .75mm cable, and a clamp which is o modified spring clamp, and also a 2m length of cable which is soldered into the earth pin of a 13A plug to measure the bonding resistance of mains sockets. The value of 100 milliohms has been considered a standard, ( in fact it was THE standard then) since 1989, as was the touch voltage of 10 millivolts.
The value of 100 milliohms has been considered a standard, ( in fact it was THE standard then) since 1989, as was the touch voltage of 10 millivolts.
Standards evolve continually to match the 'state of the art'.
The value of 100 miliohms for mains socket-outlets originated from the calculation to keep the simultaneous voltage between accessible conductive parts below 25V a.c. on medical IT systems and made the assumption that a 20A type B MCB was fitted. This assumes a fault in another item after a first fault to earth has already occurred, since the IT system then becomes a TN system.
The value of 10mV was only ever mentioned in guidance documents or informative notes and never mentioned in a official standard. This was just as well as it made a false assumption that the electrical installation earth needed to meet the CF applied parts touch leakage limits. Clearly this was quite flawed, as even a CF rated medical device is allowed 100mV touch leakage (measure across a 60601-1 human body model). Also the medical IT systems allowed 500uA leakage current so making a mockery of the 10mV (10uA) limit. Measuring a voltage using a high impedance voltmeter, as often suggested, will not give a true idea about the leakage current that may affect a patient, especially for parts isolated from Earth and when using long measuring leads, due to induced pick up.
Currently there is no requirement in the UK to measure any touch leakage for the electrical installation, such as mains socket-outlets. This can only be applied to medical equipment in accordance with BS EN 62353.
For more details please refer to IEC 60364-7-710 document, I think UK no more using MEIGEN document anymore, Please advice if I am wrong. RCDS are not allowed with IT system according to IEC 60364-7-710, but all the information is comeback to IEC60601-1.