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Surge protection

Surge ProtectionInductive loads

Inductive loads produce very large voltage spikes, which will cause reed switch failure. Typically inductors such as relays, solenoids, and motors will cause voltage spikes of several hundred to thousand volts which will severely damage contacts and greatly shorten life. This spike happens when the contact opens causing the electrical field to collapse, generating what is known as counter emf. Counter emf must be reduced to safe levels in order to achieve rated life and load specifications.

Incandescent lamp loads

Inrush current for incandescent lamps is 10 to 15 times the rated current as specified by the manufacturer. High current draw happens when the lamp is turned on. The lamp has low resistance that increases with time 
(approximately 1 second.)


Switch failure will occur without proper surge protection.  It is your responsibility to incorporate this protection. Surge protection devices should be mounted within 10" of the load or switch.

Examples of protection devices

1) D.C. inductive loads use a diode with a reverse breakdown voltage at least 10 times the applied voltage, and a forward current minimum of the load current draw.

2) A.C. inductive loads use a varistor, MOV, or r-c network rated for applied voltage. As a guide if using r-c network 0.5 to 1 ohm per applied volt, and .05uf to 1uf capacitor per 1 amp. current, with a minimum of 0.5uf. Use AC type non-polarized capacitors with a minimum breakdown voltage higher than the supplied AC peak to peak voltage.

3) Incandescent lamps need either a series or parallel resistor. Follow the respective equation for sizing the resistor.

E = Applied voltage
Imax = E divided by switch wattage rating
R, lamp = Cold lamp resistance with ohmmeter
R = Value of resistor needed

115 VAC R/C network
P/N QA 115