Overload Relay
Protection
The motor's bodyguard: models motor heating and disconnects the contactor coil when sustained current says the windings are cooking. Trips on overload — deliberately slow, so it ignores normal starting inrush.
What it looks like
Overload Relay
A typical protection — exact shape, colour and markings vary between manufacturers.
Types & variants
Key specs
Current setting range
AAdjustable dial — set to the motor's nameplate FLC.
Trip class
Class 10 trips in 10 s at 6× current; class 30 tolerates long starts.
Phase-loss protection
Detects a lost phase (single-phasing) — a top motor killer.
Auxiliary contacts
95-96 NC breaks the contactor coil; 97-98 NO for alarms.
Laws that govern it
- Joule's law of heatingH = I² × R × t
The bimetal is a thermal analogue of the motor winding: both heat as I²t, so the relay trips on the same curve the insulation would cook on.
- Laws of resistanceR = ρ × L / A
Bimetal strips work because two metals with different expansion (and their resistance heating) bend predictably with temperature.
- Watt's lawP = V × I = I²R = V²/R
Class 10 vs class 20 is how many seconds of that heating it tolerates — matched to whether the load starts quickly or slowly.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Amp dial, trip class, terminal numbers (95/96/97/98), test and reset buttons.
Standard values
Sold in overlapping ranges (e.g. 1.6–2.5 A, 2.5–4 A ...) that clip onto matching contactors.
How to choose
Range that puts nameplate FLC mid-dial, trip class matching start duration (high-inertia loads need class 20/30), phase-loss sensitive for three-phase, manual reset so trips get investigated.
Pinout & package
Clips under its contactor; power in from contactor T-terminals, motor out; control terminals 95-96 wired in series with the coil.
Example circuits
- DOL starter: overload's 96 in series with contactor coil
- 97-98 wired to a trip indicator lamp
- Class 30 on a loaded conveyor start
Common failures
Nuisance trips from a setting below FLC or high ambient heat, welded mechanism after clearing a hard fault, and the human failure of cranking the dial to max to 'stop the tripping'.
How to test
Press TEST — the 95-96 contact should open (contactor drops). Verify the dial matches motor nameplate. True trip-time verification needs current injection.
Substitutes
Same range/class replacement matched to the contactor; electronic versions add accuracy and phase-loss detection in the same frame.