Potential monitoring device – Type 8521

As per DIN EN 50122 part 1 / VDE 0115 part 3


As protection from dangerous body currents generated from contact voltages/tapping voltages, maximum voltages have been stipulated depending on the effect time. On exceeding the maximum voltages and effect time, safety measures such as short circuit or switching off have to be initiated.

The potential monitoring device type 8521 has been developed to register these maximum voltages taking account of their effect time, and to trigger switching measures.


The potential of the contact points to be monitored is supplied to terminals 1 and 4. Polarity, DC or AC voltage is irrelevant for the connection.

K2 and K3 take into account the voltage/time values for the “normal” range. Relay K2 and K3 are working in operating circuit mode.

The potential monitoring device already takes account of a time workload of > 150 ms for the subsequent switching measure for a DC of 420 V and an AC of 295 V.
Caution: for possible higher touchable voltages! The time required for switching off or short-circuiting must always be included in the planning (should not be longer than 150 ms). If registration and reaction times are allowed longer than DIN EN 50122, additional measures (e.g. voltage-limiting devices) are to be provided.

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