Electrical safety: Circuit breakers

by Oct 9, 2018Electrical Safety

Electrical safety: 

Circuit breakers


This blog aims to explain the functioning of circuit breakers and the impact of adjusting the instantaneous pickup on incident energy levels. The information that follows is limited to equipment supplied by a 600V source.

There are a multitude of breakers, some breakers can be adjusted to allow variation of the following tripping parameters: ‘’long-time, short-time and instantaneous’’. Other breakers only allow variations of the instantaneous trip or do not allow modification of any parameters.

For example, molded-case circuit breakers with an amperage drawof less than 100 A generally do not have adjustments. Molded-case circuit breakers with an amperage drawof 100 A or more only allow the adjustment of the instantaneous trip. Low voltage power circuit breakers with solid state trip units allow adjustment of all the parameters: « long-time pickup and long-time delay, short-time pickup and short-time delay, instantaneous pickup ».

The following figure shows these different parameters and how to locate them on the breaker’s time-current coordination curve. The curve can be found directly on the breaker’s technical data sheet.

Figure 1: Breaker instantaneous trip, short time and long time

Instantaneous trip time is an important parameter when a task needs to be performed under voltage on an equipment where no data on the incident energy level exists. According to CSA Z462-18, during live work on 600V equipment where the trip time is under 30ms and the short-circuit current is at most 25 kA for panels or equipment, and at most 35kA for motor control centers, category 2 arc flash PPE would be acceptable. However, in the event where the instantaneous trip time is over 30ms,say 330 ms, with a short circuit current of at most 42 kA for a motor control center, or 500 ms and a short circuit current of at most 35 kA for a switchgear, category 4 PPE would be required.

An incident energy study is more reliable to determine the right PPE to wear for live work. It also allows verification that the breakers are well adjusted. It happens within companies that motors are not well maintained or are exposed to dirt or are overused. This misuse causes premature aging. Therefore, the motors could require higher start currents than originally and cause unexpected trips of protective devices. Maintenance personnel could adjust the breaker’s instantaneous trip, but this modification could increase the protective device trip time and thus increase the incident energy level. The three following figures demonstrate the impact of increasing instantaneous trip on arc fault current.

Situation 1:

  • Instantaneous trip: 30 000 A
  • Protective device tripping time: 50 ms
  • Arc fault current: 54.135 kA
  • Incident energy level: 10 cal/cm2

Situation 2:

  • Instantaneous trip: 36 000 A
  • Protective device tripping time: 69 ms
  • Arc fault current: 46.015 kA
  • Incident energy level: 11.6 cal/cm2

Situation 3:

  • Instantaneous trip: 45 000 A
  • Protective device tripping time: 140 ms
  • Arc fault current: 46.015 kA
  • Incident energy level: 23.5 cal/cm2

Figure 2: Situation 1: Instantaneous trip = 30 000 A

Figure 3: Situation 2: Instantaneous trip = 36 000 A

Figure 4: Situation 3: Instantaneous trip = 45 000 A

To know more about this topic or to obtain our incident energy study services, don’t hesitate to communicate with our team of experts!


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Intervention Prévention Inc.

Intervention Prévention concentrates its operations in the field of work safety, offering specialized services following Standards CSA Z462 – Workplace Electrical Safety, CSA Z460 – Control of Hazardous Energy: Lock-out and Other Methods, and CSA Z432 – Safeguarding of Machinery.


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