Follow these steps to select a suitable relay according to the load type, operating current, inrush current and required contact configuration.
Step 1: Determine the Load Type
Identify whether the relay will control a resistive, inductive, motor, lamp or capacitive load. Relay contact ratings are normally based on resistive loads, while other load types require additional derating.
Step 2: Check the Load Current and Voltage
Confirm the steady-state operating current and switching voltage of the connected equipment. Both values must remain within the relay contact specifications.
Step 3: Apply the Correct Load Derating
For inductive loads, the allowable current should not exceed 30% of the relay’s resistive-load rating. Motor loads should not exceed 20%, lamp loads should not exceed 10%, and capacitive loads should not exceed 5%.
Step 4: Calculate the Inrush Current
Check the starting or inrush current instead of considering only the steady-state current. Resistive loads may produce approximately 1 times the steady-state current, while inductive, motor, lamp and capacitive loads may produce considerably higher starting current.
Step 5: Select a Suitable Rated Contact Current
Select a relay with sufficient current capacity and operating margin. For reliable operation, the actual steady-state contact load should generally remain within approximately 60%–70% of the relay’s rated load current.
Step 6: Select the Contact Form
Choose the required contact arrangement: Form A for normally open contacts, Form B for normally closed contacts, or Form C for changeover contacts. The contact form must match the switching logic of the application.
Step 7: Confirm the Final Relay Specification
Verify the coil voltage, contact rating, contact form, load type, switching frequency and installation conditions before ordering. Contact HWRELAY for model matching or OEM/ODM relay customization when the application has special requirements.
Unless otherwise specified, all load currents indicated on the Hangwang relay case are for resistive loads. Under the rated switching voltage, the allowable current of inductive loads should not exceed 30% of the rated resistive loads, the allowable current of motor loads should not exceed 20% of the rated resistive loads, the allowable current of lamp loads should not exceed 10% of the rated resistive loads, and the allowable current of capacitive loads should not exceed 5% of the rated resistive loads. In order to ensure the reliability of the product, it is recommended that the contact load current value should not exceed 60% – 70% of the steady state current value under the rated load of the product.
| Nature of the load | Inrush current |
| Resistive load | 1 times the steady-state current |
| Sodium lamp load | 1-3 times the steady-state current |
| Mercury-vapor lamp load | 3 times the steady state current |
| Inductive load | 3-5 times the steady-state current |
| Motor load | 3-5 times the steady-state current |
| Voltage load | 3-5 times the steady-state current |
| Incandescent lamp load | 3-5 times the steady-state current |
| Capacitive load | 3-5 times the steady-state current |
| Name | Domestic code | International code | Structural form |
| Conversion type | Z | C |  |
| Normally open | H | A |  |
| Normally closed | D | B |  |
Switching type:
The center is a moving contact, coil is not energized, the moving contact and one of the static contact is disconnected and the other one is closed, after the coil is energized, the moving contact will move, so that the original disconnected into a closed state, the original closed into a disconnected state, to achieve the purpose of conversion.
Normally open type:
The contacts are open when the coil is not energized and closed when energized.
Normally closed type:
The contacts are closed when the coil is not energized, and the contacts are closed when energized.
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