With the use of AFDD’s becoming more and more popular in recent times one question we keep getting asked is if we can provide the maximum Zs values for AFDD’s (Arc Fault Detection Device).
Values of maximum Zs (earth loop impedance) are applicable to over-current protection devices.
Combined devices which offer Arc Fault Detection AND over-current protection are generally manufactured to two sets of standards, for example an AFDD conforming to BS EN 62606 are recommended as a means of providing additional protection against fire caused by arc faults in AC final circuits, where as an MCB to BS EN 60898 offers protection against over-current.
Where a combined device is used such as above, the over-current BS EN part of the combined device is used to determine the maximum permitted Zs value of the AFDD combined unit.
For further information regarding AFDD’s see this post AFDD’s Arc Fault Detection Device – Do we need them? for details on where and when we need to install AFDD’s in Amendment 2 of BS 7671 18th Edition.