Touch Current: Which Limit Applies and Why the Network Matters

Touch current is the current that flows through a person who touches an accessible conductive part. The standard sets limits that depend on the nature of the supply and on the connection of the equipment, because the current that can flow through a person is determined by the whole circuit, not by the instrument alone.

The measuring network

The current is not measured with a plain ammeter. A specified network, representing the impedance of the human body, is connected between the part and earth or between the part and the other pole of the supply, and the voltage across a defined element of that network is measured. Different networks apply to different situations, and using the wrong one produces a value that cannot be compared with the limit.

Normal and fault conditions

Limits are given for normal operation and for single fault conditions. The fault condition limits are higher, which reflects the probability of the fault coinciding with a person touching the part. Testing only in normal condition leaves the fault case unassessed, and that is where most real shocks originate.

What raises the current

EMC filter capacitors to earth, long input cables with capacitance to earth, and transformer inter-winding capacitance. An instrument that measures comfortably within limits on a bench can exceed them when connected to a long supply lead, which is why the test configuration has to be defined and recorded.

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• Temperature Rise: Limits, Ambient Assumptions and the Parts People Forget