Why Does an Overload Relay Trip Repeatedly? Causes and Troubleshooting

Overload Relay tripping again and again ??

That can be serious signal !

or

That can be just an error in control signal !

The overload relay tripping need to be figure out. Otherwise, Serious outcome can occur.

In this post, We are going to discuss the repeated tripping of overload relay.

Overload Relay

Overload relay is a type of safety device, To protect Electrical motor from “overload” and “short circuit” conditions. Some of the overload relay provides protection from “single phasing“.

The line current of motors passes through overload relay in a series manner. In “overload” or “short circuit” condition, High current through these overload relay produces a change in control signal. That signal is used to trip or as an alarm for motor safety.

How does this overload relay works

In Overload relay, There are 2 circuits working parallelly that are :

1. Bimetallic thermal tripping ( Overload tripping or Delayed tripping )
2. Electromagnetic tripping ( short circuit tripping or Instant tripping )

An image showing the internal circuit of thermal overload relay. In this, Current flow, is marked from 1 to 2 in yellow marking. Further; current flow, is marked from 2 to 3 in green marking.

1. The path 1 to 2, is bimetallic thermal element. When high current(more than rated current) flow through this bimetallic thermal element for some time. Due to I²R loss, This thermal element get heated and get bent in overload condition. The generated bend is further used to move the trip circuit.
2. The path 2 to 3, Electromagnetic circuit. When rapid current flow through this electromagnetic circuit in short circuit conditions. Due to generated magnetism, This electromagnetic coil get bent in short circuit. The generated bend is further used to move trip the circuit.

Both the phenomenon trips the one common circuit which is further connected to NO and NC contacts. In overload or short circuit conditions, These contacts changes their states.

Overload relay tripping

Overload relay tripping should be in such a way that In normal working condition, It should not trip. While whenever the load exceeds the rated or desired set limit, it get trip.

2 faults conditions : “Overload “and “short circuit“.

Overload is slow process. While short circuit is rapid process. Both the cases, Current increases more than the rated current. Higher the current lower will be the trip time !

For better understanding about the thermal overload tripping, Next we are discussing the current Vs time characteristics of ABB make thermal overload relay TF42. Relay’s current range is : 16 to 20 Amps

In the characteristics, x-axis is multiplies of rated current, Ie. While y-axis is Time(Sec.). Both the curves are shown, One for 3 pole(normal condition) While second for 2 pole(single phasing) condition.

When current reaches 3 time the rated current (rated current is the selected current of thermal overload relay), In 3 pole conditions it trips in 19.9 sec. In 2 pole (single phasing), It trips in 16.4 sec. Single phasing situation trips more rapidly comparatively.

A table and graph of current and tripping time are shown below :

It is seen that As rated current across the thermal overload relay increases, Its tripping current decreases proportionally.

Multiplies of rated current, Ie	Time (Sec.) Curve for 3 poles	Time (Sec.) Curve for 2 poles (Single phasing)
3*Ie	19.9	16.4
5*Ie	8	6.8
7.2*Ie	4.6	3.9

Let suppose selected range is 20 Amps. Than, Ie becomes 20

Multiplies of rated current * Ie	Time (Sec.) Curve for 3 poles	Time (Sec.) Curve for 2 poles (Single phasing)
3*20=60	19.9	16.4
5*20=100	8	6.8
7.2*20=144	4.6	3.9

Causes of overload relay tripping

• Damaged insulation of motor winding
• Damaged insulation of cable
• Short circuit at motor side
• Short circuit in cables
• Overloaded motor
• Single phasing or phase imbalance.
• Incorrect overload relay current setting.
• Loose connections and faulty contacts.
• Frequent starting or excessive starting time.
• Mechanical problems in the driven machine.
• Faulty overload relay or incorrect selection.
• False wiring

How to Set an Overload Relay Correctly

Let suppose, 1.1KW-1400 rpm Induction motor with 415V star connections. Its rated current is 2.58 Amps. Rated current is the limit under which motor works precisely under the safe limit. Under this limit, Motor can utilize its maximum useful life.

Above the rated current, Motors noise and vibration increases. Its winding insulation falls under the stress. Winding insulation can permanently get damaged if “exposure is long” or “current level is much higher“.

That motor must run under the rated current. Further more, Selection of current also depend upon characteristics of motor and system behavior where it is connected. For this motor, Thermal overload relay with current rating of 2.58 is required.

There are different companies making thermal overload relay with different ampere ranges such as 1.5 to 2.5, 2.5 to 4, 1.9 to 2.8. Any of the thermal overload relay can be used, While 2.58 can be considered as its limit.

Troubleshooting overload relay tripping

Tripping is very important, It is like an alarm. Even a single tripping is important to consider. Next, We are discussing, How to rectify the fault and eliminate the faulty element.

1. After the trip, Check whether overload relay trip or something else ? If not trip, Check by run the motor again. Monitor the ampere of motor.
2. If overload relay is trip than First disconnect the motor terminals from thermal overload relay.
3. Check the voltage present at input side of contactor.
4. Reset the thermal overload relay.
5. Run the starter, Check the voltage at each output terminal of thermal overload relay.
6. If Voltage available and starter is holding than thermal overload relay and control circuit working fine . If not, Than control circuit is faulty. Check the total circuit from NC stop contact to NO start contact, Contactor’s NO contact, Contact’s coils and contacts of thermal overload relay.
7. If control circuit is holding working fine than check the motors terminals after disconnecting the motor terminal from thermal overload relay.
8. First, Test the insulation of motor with megger or digital multimeter.
9. Check the resistance between each coil of motor. The resistance value should be identical with minor tolerance value. If insulation fails or resistance value varies than motor is faulty, Change the motor. (if possible refer the resistance value with its technical data sheet.
10. If insulation is fine and resistance values are identical, Check for the bearing of the motor.
11. Check the bearing condition by rotating the rotor. If mechanical load is connected disconnected the load first. If bearing is faulty; changed the motor or replace the bearings.
12. When starter is good and motor is also good. Connect the starter to motor.
13. Run the motor at no load. Observe the line current, It should be lower than its rated current. If current is much higher than rating current than disconnect the motor and check the motor internally after opening.
14. Observe noise and vibration of motor. If all is fine, proceed to put load on the motor.
15. Add some load on the motor. Observe the line current, It should be lower than the motor rated current. If current is much higher than rated current check the mechanical load.

Can You Reset an Overload Relay Repeatedly?

As discussed, Repeated thermal overload relay can due to faulty overload relay. But the chances of faulty thermal overload relay is very less. Most of the cases, There are other chances.

If motor or cable is faulty, Then fault current produces excessive burden on the entire circuit. frequent reset, Can damage the thermal overload relay or it can damage the motor permanently. Moreover, repeated reset produces the stress on entire back end distribution circuit. Due to one faulty element, Entire assembly can shut.

Quick trouble shoot of overload tripping

Observation	Possible cause	What to check
Trips after running for some time	Motor overload or excessive mechanical load	Motor current and driven machine
Trips during starting	Excessive starting time or incorrect setting	Starting current, acceleration time and relay settings
Trips with phase-current imbalance	Supply problem or faulty connection	All three phase currents and connections
Trips even under normal load	Incorrect setting, unsuitable relay or faulty relay	Nameplate current, relay specifications and relay condition

These are possible causes, not definitive diagnoses. The actual cause must be confirmed through measurements and inspection.

Important technical point: Do not assume every overload relay should be set exactly to the motor’s nameplate full-load current. The appropriate setting depends on the relay type, manufacturer’s instructions, motor starting conditions and applicable protection requirements.