Supplementary equipotential bonding in group 2 medical locations

There are two things to consider.

The first is robustness, both mechanical and electrical - the wire has to be chunky enough it is  not going to snap if pulled by accident during cleaning/maintenance and it needs to be large enough to carry any current that may flow through it during a credible fault condition. (noting that credible fault may be very short lived - the time it takes the breaker to trip may be very short, so the cable can take much more than it;s steady state current rating without failure - but not for long.)


The second is actually the main function of a bonding conductor., which is to be low enough resistance that the voltage drop along in any normal use case (not all faults) it is small enough not to be dangerous.

This has to be converted into a maximum resistance, which in this case has been decided by the spec writers for you , as 0.2 ohms. This can be seen as  really  a couple of volts if ten amps was flowing. If you had one part of the patient that sort of voltage offset from another ,they are unlikely to come to harm. Actually for non medical use up to about 50V between body parts is considered to be usually OK, if a bit tingly,  for glancing contact with bare skin, but for medical facilities where the patient may not be well, may be wet, may not be fully clothed, may not be able to  flinch and self disconnect etc, the limits are set tighter.


So how do we translate 0.2 ohms (or any other number) into a cable size. Well we need to look at the resistance of copper (or whatever we are using, usually copper.) There are big tables of this but a 'get you close' rule of thumb is to remember one number - sixteen .


A one meter length of copper wire, of 1mm2 cross section, is 16 milliohms (0.016 of an ohm) when at room temp.

A longer length has more resistance pro-rata, say 10m of 1mm sq would be 160 milliohms.

A larger cross section would be less, also pro-rata, so a 4m length of 4mm2 is the same resistance of 1m length of 1mms, or a 300m length of 300mm2. (that's a sort of street main size..)

If you look at the tables you will see it is not quite true, but is good enough for a quick sum in the head to see if you are safely in spec by a factor of 2 or more, or if you  need to look it up and do it properly, or indeed if there is no way it could ever meet the spec without even reaching for a meter. to check.

Note that is for cold copper, and to re-use that rule of thumb for estimating voltage drop in poer cables should normally be calculated assuming  hot conductors  (70C or 90C) and is closer to 18 or 19 milliohms per metre of 1mm2.


Hope this helps.

Mike

There are two things to consider.

The first is robustness, both mechanical and electrical - the wire has to be chunky enough it is  not going to snap if pulled by accident during cleaning/maintenance and it needs to be large enough to carry any current that may flow through it during a credible fault condition. (noting that credible fault may be very short lived - the time it takes the breaker to trip may be very short, so the cable can take much more than it;s steady state current rating without failure - but not for long.)


The second is actually the main function of a bonding conductor., which is to be low enough resistance that the voltage drop along in any normal use case (not all faults) it is small enough not to be dangerous.

This has to be converted into a maximum resistance, which in this case has been decided by the spec writers for you , as 0.2 ohms. This can be seen as  really  a couple of volts if ten amps was flowing. If you had one part of the patient that sort of voltage offset from another ,they are unlikely to come to harm. Actually for non medical use up to about 50V between body parts is considered to be usually OK, if a bit tingly,  for glancing contact with bare skin, but for medical facilities where the patient may not be well, may be wet, may not be fully clothed, may not be able to  flinch and self disconnect etc, the limits are set tighter.


So how do we translate 0.2 ohms (or any other number) into a cable size. Well we need to look at the resistance of copper (or whatever we are using, usually copper.) There are big tables of this but a 'get you close' rule of thumb is to remember one number - sixteen .


A one meter length of copper wire, of 1mm2 cross section, is 16 milliohms (0.016 of an ohm) when at room temp.

A longer length has more resistance pro-rata, say 10m of 1mm sq would be 160 milliohms.

A larger cross section would be less, also pro-rata, so a 4m length of 4mm2 is the same resistance of 1m length of 1mms, or a 300m length of 300mm2. (that's a sort of street main size..)

If you look at the tables you will see it is not quite true, but is good enough for a quick sum in the head to see if you are safely in spec by a factor of 2 or more, or if you  need to look it up and do it properly, or indeed if there is no way it could ever meet the spec without even reaching for a meter. to check.

Note that is for cold copper, and to re-use that rule of thumb for estimating voltage drop in poer cables should normally be calculated assuming  hot conductors  (70C or 90C) and is closer to 18 or 19 milliohms per metre of 1mm2.


Hope this helps.

Mike