1. When the rated voltage is applied to the electromagnetic coil, it can pull in normally and reset normally after power failure. When the coil is not powered on, the normally closed contact shall be well connected and the normally open contact shall be kept open. After the coil is electrified, the normally open contact shall be well connected and the normally closed contact shall be ensured to be disconnected. Such a relay is good.

2. Use the resistance block of the multimeter to measure the resistance between the normally closed contact and the moving point. The resistance value should be 0; The resistance between the normally open contact and the moving point is infinite. This can distinguish which is normally closed contact and which is normally open contact. R of multimeter × Measure the resistance value of normally closed contact at gear 1, which is normally 0; When the armature is pressed, the resistance of the normally closed contact should be infinite. If there is a certain resistance or infinite value of a group of normally closed contacts when the armature is not pressed, it means that the group of contacts has been burnt out or oxidized.

3. The coil resistance of electromagnetic relay is generally 25 Ω ~ 2K Ω. The coil resistance of electromagnetic relay with low rated voltage is lower, and that of electromagnetic relay with high rated voltage is higher. Available multimeter R × Measure the resistance of the relay coil at 10 Ω to judge whether the coil has open circuit. If the measured resistance is infinite, the coil has been broken; If the measured resistance is much lower than the normal value, there is a short circuit fault inside the coil. If there is a local short circuit in the coil, it is not easy to find it by this method.