Protecting contacts from arching

Why do relay and other switching contacts seem to wear out faster than expected? This basic problem has an explanation that can often be reduced down to one word – inductance.

This problem is surprisingly common on board superyachts and yet quite a few engineers I’ve worked with are baffled what to do about it.

Inductive loads such as motor starters and solenoids, fed by a pair of contacts from a power source, whether it’s AC or DC, require the use of some type of surge suppression to protect the contacts.

This is because when the circuit to an inductive load is opened, much of the energy stored as the magnetic field in the inductance must go somewhere as the field collapses and will be dissipated as arcing at the contacts, unless some alternative means of energy absorption is provided.

Switching inductive loads without surge suppression can significantly reduce the life expectancy of relay contacts. By adding a suppression device directly across the coil of the inductive devices, you prolong the life of the output circuits. You also reduce the effects of radiated voltage transients and prevent electrical noise from radiating into system wiring.

The above diagram shows a contact driving an inductive load with a suppression device added. We recommend that you locate the suppression device as close as possible to the load device, as shown.

Suitable suppression methods for inductive load devices include an RC network only for AC loads (left suppressor symbols in the above diagram); or for DC loads, use a suitable rated diode (right); or a varistor (or VDR, or bidirectional Transorb), can be used for both AC and DC loads. These components must be appropriately rated to suppress the switching transient characteristic of the particular inductive device.

For DC circuits only, the semiconductor diode may be connected across the inductive load (see above diagram, note diode points up, towards positive terminal of the inductive load) so that it blocks the applied voltage at contact closure but allows the stored energy in the load to recirculate, ‘freewheel’ through it at contact opening. Hence, this diode is usually referred to as a ‘freewheel or ‘flywheel’ diode.

[(*) For much more in-depth information and training on circuit diagrams and diodes, see module 2 of our course: Electrical Control Systems for superyacht engineers.]

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