What should be considered when choosing a relay current rating?
Looking at a single ampere figure on the label is not enough. The relay's continuous current rating, the type of voltage it switches (AC/DC), the load's inrush current and the ambient temperature are evaluated together. With inductive loads, inrush current can be several times the continuous current. The voltage drop measured across the contact under load should stay below 0.2 V; above 0.5 V means the contact is worn.
Make the relay selection under four headings.
Continuous current. Measure the load's normal operating current, do not trust the label. On frequently switched lines, leave one step of margin above the measured value.
Voltage type. An AC rating cannot be used for DC. In DC the current does not pass through zero, so the arc does not extinguish; the same contact interrupts a markedly lower current in DC. For a DC load, always look at the DC rating.
Load character. Motors, coils, lamps and devices with capacitive inputs draw a high inrush current at start. That current wears the contact at the moment of closing and increases the risk of welding over time. A suitable contact material and one step up in size extends life.
Environment. Temperature affects both the coil resistance and the contact rating; vibration, by making the contact bounce, lengthens arc duration. In a vibrating installation, fix the relay body firmly and preferably use a sealed type.
Check after commissioning: measure the voltage across the contact terminals while the load is running. Below 0.2 V is healthy, 0.2-0.5 V is borderline, and above 0.5 V means the contact is worn or its socket is loose and it must be replaced. This measurement catches the cases where a relay appears to keep working but does not pass enough voltage to the load.
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