Why should I derate the relay current rating on an inductive load?
Because the ampere figure in the catalogue is mostly measured under resistive load (AC-1) conditions. With inductive loads such as contactor coils, solenoids and motors there is both a start-up current of about 10 times the running value and an arc at the moment of breaking. For this reason the common practice is to apply a significant derating, and even to use only 20-40 percent of the resistive rating.
The ampere figure given for a relay contact tells you under which conditions and how many times the contact can safely make and break. That condition is usually a resistive load.
An inductive load imposes two additional stresses:
- The moment of closing: the coil or motor draws a start-up current of up to about 10 times the running current. The contact surface is stressed as it closes against this current.
- The moment of opening: the energy stored in the inductance is released as an arc. The arc transfers contact material and over time welds the contact or leaves it non-conducting.
Load categories define this difference: AC-1 covers resistive and lightly inductive loads with a power factor above 0.95, while AC-15 covers inductive control loads such as contactor coils. If only an AC-1 rating is given for a contact, it is a widespread engineering approach to assume the AC-15 capacity is on the order of 15-20 percent of that figure.
The practical consequence: when calculating the load, base it on the derated value rather than the nameplate current; suppress the breaking arc with a snubber or freewheeling diode; switch large loads with a contactor rather than a relay. Applied together, these three measures extend contact life many times over.
Full article: Wireless remote control installation: pairing, relays, safety