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The protection element also provides a non-sensitive area to avoid instability due to small  asymmetries and unbalances that can generally be present in the network system. This  condition is represented by the characteristic reproduced below, where the hatched area is the tripping zone.

Description:

At current values greater than Ic, the SCPD reacts quicker than the overload 

relay. At current values less than Ic, the overload relay reacts quicker. Articles 

110 and 430 of the NEC provide guidance in the selection of the SCPD to 

facilitate coordination of the components of a motor branch circuit (i.e. 

location of point Ic).

Withstand Ratings

Equipment withstand ratings are linked to branch circuit protection. The 

same parameters that affect the trip point of a given protective device also 

contribute to how much (or how little) let-through energy the device may be 

exposed to and still function after the clearing of the fault. Withstand does not 

explicitly show up in Figures 1 or 2. Traditional melting alloy and bi-metal 

overload relays have been the “weak link” in motor branch circuit withstand 

ratings. Since these devices employ sensing elements directly in the current 

path, electrical faults leading to mechanical stresses are a concern. These 

devices typically contain small mechanical parts than can quickly become 

out-of-spec when exposed to let-through energy exceeding their withstand 

capability. If the coordinated protection for the circuit operates properly (and 

the SCPD protects the circuit), the motor and the controller will be protected. 

The withstand rating of a branch circuit must account for the withstandability 

of the lowest rated component in the circuit.

Thermal Overload Relay

In spite of being relatively simple and inexpensive, thermal overload relays 

are very effective in providing motor running overcurrent protection. This is 

possible because the most vulnerable part of most motors is the winding 

insulation and this insulation is very susceptible to damage by excessively 

high temperature.

Being a thermal model of a motor, the thermal overload relay will produce a 

shorter trip time at a higher current similar to the way a motor will reach its 

temperature limit in a shorter time at a higher current. Similarly, in a high 

ambient temperature, a thermal overload relay will trip at a lower current or 

vice versa allowing the motor to be used to its maximum capacity in its 

particular ambient temperature (if the motor and overload are in the same 

ambient).

Once tripped, the thermal overload relay will not reset until it has cooled, 

automatically allowing the motor to cool before it can be re-started.

NOTE: The overload relay must be used in conjunction with a contactor. The 

overload relay has no power contacts and cannot disconnect the motor by 

itself. The control circuit contact must be wired in series with the coil of the 

contactor so that the contactor will de-energize when an overload occurs.

Square D manufactures three types of overload relays, the melting alloy, the 

bimetallic, and solid state. In some types, the bimetallic is available in both 

non-compensated and ambient temperature-compensated versions. In both 

melting alloy and bimetallic, single element and three element overloads are 

available. Solid state overloads are discussed on Page 5.

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