Relays are among the most widely used switching devices in electrical and electronics systems — from a simple 12 V automotive relay to the protection relays guarding transformers and feeders in a 33 kV substation. When a relay fails, the symptom is often a circuit that will not operate, a motor that will not start, or — more dangerously — a protective relay that fails to trip during a fault. Testing a relay systematically with a multimeter (and, for protection relays, a dedicated relay test set) is a skill every electrical technician should have.
Relay Basics — What You Are Testing
Every electromechanical relay has two electrically isolated circuits: the coil (the electromagnet that does the switching) and one or more sets of contacts (the actual switching elements). The coil has terminals A1 and A2; applying rated voltage across them energises the relay. The contacts are described as Normally Open (NO — open when coil is de-energised), Normally Closed (NC — closed when de-energised), and Common (COM — the pivot terminal shared between NO and NC).
Test 1 — Coil Resistance
Set your multimeter to the resistance (Ω) function. Place probes across the coil terminals (A1 and A2). A healthy coil shows a specific resistance that you can compare against the datasheet. An open coil shows infinite resistance (OL). A shorted coil shows near-zero resistance.
| Coil voltage | Typical coil resistance | Reading if open | Reading if shorted |
|---|---|---|---|
| 5 V DC | 25 – 100 Ω | OL / ∞ | < 1 Ω |
| 12 V DC | 100 – 400 Ω | OL / ∞ | < 1 Ω |
| 24 V DC | 400 – 900 Ω | OL / ∞ | < 1 Ω |
| 230 V AC | 3 – 15 kΩ | OL / ∞ | < 50 Ω |
| 415 V AC | 8 – 30 kΩ | OL / ∞ | < 100 Ω |
De-energise before resistance testing
Test 2 — Contact Continuity (NO and NC)
With the relay de-energised (coil not powered), test the contacts using the continuity or resistance function:
Test 3 — Pick-up and Drop-out Voltage
The operate (pick-up) voltage is the minimum coil voltage at which the relay reliably pulls in. The drop-out voltage is the maximum voltage at which the relay releases. These tests verify that the relay will operate correctly across its full specified voltage range — important in battery-backed systems where supply voltage varies.
Use a variable DC power supply (or a variac for AC relay coils) and slowly increase voltage from zero while monitoring with a multimeter. Record the voltage at which you hear the relay click. This is the pick-up voltage — it should be no more than 75–85% of rated coil voltage per most relay specifications. Then reduce voltage slowly and record the drop-out voltage.
Protection relay testing requires a dedicated relay test set
Interpreting Failures
| Symptom | Likely cause | Action |
|---|---|---|
| Coil OL — no resistance reading | Open coil winding | Replace relay |
| Coil near-zero resistance | Shorted coil | Replace relay |
| Correct coil R but relay does not click | Mechanical seizure or wrong voltage | Check supply voltage; replace relay |
| NC shows OL when de-energised | Burnt or welded NC contact (open) | Replace relay |
| NO shows continuity when de-energised | Welded NO contact (stuck closed) | Replace relay — do not use |
| High contact resistance (> 5 Ω) | Corroded or eroded contacts | Clean or replace; check load current |
| Relay picks up but contacts chatter | Low supply voltage or coil failing | Check supply; replace relay |
For testing protection relays in substations and switchgear — where accuracy and logged test records are essential — CIE's relay test sets provide the precision current and voltage injection capability required. Visit our relay test equipment range or contact our application engineers to discuss your protection testing requirements.