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Posted By Topic: Bonding requirements in a Hazardous Area

May 25 2017 08:36

I am trying to clarify the requirement and minimum size for an external bonding conductor to be fitted to the metal case of any Ex device which has an external screw fitted for the purpose of fitting an bonding conductor.

I know some site requirements stipulate the fitting of an external bonding conductor and the size are as follows: Field Instruments/Junction Boxes/Cable Tray 6mm2, Field Equipment Stand/Cable Ladder/Pump/Fans 16mm2, Earth Ring Main 70mm2 etc.

AS/NZS 3000 Clause (b) directs you to clause 7.7 with regards to bonding requirements for an explosive atmosphere.

AS/NZS 3000 Clause states AS/NZS 60079.14 as the installation standard.

AS/NZS 3000 Clause states that a bonding conductor shall be no more than 0.5 ohms.

AS/NZS 60079.14 Clause 6.3.1 states Potential equalization is required for installations in hazardous areas. For TN, TT and IT systems, all exposed and extraneous conductive parts shall be connected to the equipotential bonding system. The bonding system may include protective conductors, metal conduits, metal cable sheaths, steel wire armouring and metallic parts of structures, but shall not include neutral conductors. Connections shall be secure against self loosening and shall minimise the risk of corrosion which may reduce the effectiveness of connection.

It can be seen from the above that the bonding in a hazardous area can be achieve by numerous Means including the Protective Earth, and the SWA of cables and also by physical contact of the device to the metallic part of the structure. I assume that as long as you have a maximum of 0.5 ohms between the metal case of the Ex device and the bonding ring main conductor then you have achieved the bonding requirements.

The question is this:

1: Do we still need to fit an external bonding conductor to Ex devices that have the external screw fitted for that purpose to comply with the AS/NZS 60079.14 standard or is this external bonding conductor needed only if the 0.5 ohms maximum cannot be achieved by other allowed means i.e SWA. Braid, PE conductor etc.

2: If an external bonding conductor is needed what would be the minimum size required.


May 25 2017 11:45

Seems like you have already done a good bit of research on this so the brief answer in my opinion to question 1 would be No you don't need to have a bond conductor on the fitting or enclosure if it is already Hi ,satisfactorily bonded.

The Answer to question 2 is that 60079:14 appears silent on any specific size conductor,60079:0 clause 15.3 states that the minimum is 4mm unless it is used also as the PEC. However the older standard 2381.1 stated that the Conductance between metallic parts of structures shall correspond to a cross section of at least 10mm copper .
I normally use that as the minimum but based on 60079:0 4mm is the correct answer


May 25 2017 12:21

Q1: No you don't need to fix external bonding to the device if it tests out to the correct value and is by means mentioned.

Q2: As answered by TheDon but also must consider the risk of mechanical damage. Interestingly 60079.14 does not specify so revert to 3000 which points to 60079.14 ("you spin my head right round, right round").
The Don is also correct that the previous standard 2381.1 stated 10mm2

May 25 2017 14:11

Thanks for the informative replies and I totally agree but just needed to hear it from somebody else with experience in this field.

May 25 2017 17:26

@ Hazinspector
3000 used to point to 2381.1 but now 60079 and Like you I thought WTH 60079:14 doesn't actually tell you a size , BUT reading the note in 3000 again carefully you will see that it points you to the 60079 set of standards,so at that point you have to go hunting and that is when I found the clause in 60079.0 that states 4mm