Thermal Memory

Introduction

The thermal memory is a means to simulate temperature rise and cooling caused by changes in the flow of current in the conductors.

These changes may be caused by:

• Repetitive motor starting

• Loads fluctuating near the protection settings

• Repeated circuit-breaker closing on a fault

Trip units without a thermal memory (contrary to bimetal strip thermal protection) do not react to the above types of overloads because they do not last long enough to cause tripping. However, each overload produces a temperature rise and the cumulative effect can lead to dangerous overheating.

Trip units with a thermal memory record the temperature rise caused by each overload. Even very short overloads produce a temperature rise that is stored in the memory. This information stored in the thermal memory reduces the tripping time.

MicroLogic Trip Units and Thermal Memory

The MicroLogic trip units are equipped as standard with a thermal memory.

• For the protection functions, prior to tripping, the temperature-rise and cooling time constants are equal and depend on the time delay in question:

  • If the time delay is short, the time constant is low

  • If the time delay is long, the time constant is high

• For long-time protection, following tripping, the cooling curve is simulated by the trip unit. Closing of the circuit breaker prior to the end of the time constant (approximately 15 minutes) reduces the tripping time indicated in the tripping curves.

Short-time Protection and Intermittent Faults

For the short-time protection function, intermittent currents that do not provoke tripping are stored in the MicroLogic memory. This information is equivalent to the long-time thermal memory and reduces the time delay for the short-time protection. Following a trip, the short-time tsd time delay is reduced to the value of the minimum setting for 20 seconds.

Ground-fault Protection and Intermittent Faults

The ground-fault protection implements the same function as the short-time protection.