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Overcurrent Conditions in Electrical Circuits

Safety and reliability of electrical systems are the primary goals in your building's electrical distribution system. To prevent fire hazards, there are two types of overcurrent conditions that need to be minimized.

Overloads in Electrical Systems

Fuses will blow before the conductors pose a fire hazard.

The fuses are designed to blow before the conductors can pose a fire hazard to the building.

Electrical overloads happen when more current is drawn for which the branch can handle.

We all know that plugging too many appliances into a receptacle will trip a breaker. This is the typical safety mechanism in homes and commercial buildings to prevent the conductor from overheading and creating a fire hazard in the wall.

There is also another overload condition that mainly occurs in industrial installations, mines and pharmaceutical plants. Motors usually do not start at the same time but the probability of simultaneous starting and running of motors can eventually occur.

When electric motors start, they can draw well beyond the normal operating current usage (full load ampacity) to overcome inertia and counter-EMF by the armature as it rotates.

When more than one large motor starts at the same time, high in-rush currents occur, higher than which the conductor were sized.

If either type of overload occurs for long enough, the circuit breaker will trip or the fuse will blow to automatically cut off this fire hazard.

Motor starting is a specialized branch in electrical engineering that focuses in:
  • Providing enough power
  • Reducing in-rush current
  • Coordinating motor starting sequences
  • Increasing the operational life of the motor
  • Reducing maintenance costs for the owner
They are also used to stop motors, reverse directions and protect them from overload conditions.

Short-Circuit Current

Rodents causing overcurrent problems in electrical systems

A short is caused when two conductors come into contact with one another. In this case, it was a squirrel that connected the two conductors (Fort McMurray).

As electrical engineering consultants, we were called to investigate an electrical problem at a commercial operation.

After discussing the failures with the maintenance guy and tracing the complete circuit, it was found that rodents had damaged the insulation on electrical cables. This resulted in phase-to-ground faults which immediately tripped the breakers.

Large amounts of current flowed in the conductors and the protection devices kicked in. Other types of we investigated were shorts to:

  • Another supply wire (a phase-to-phase short)
  • The return wire (a phase-to-neutral short)
  • Any metal connected to earth (a phase-to-ground short).

Of these, the phase-to-earth fault are the most common. Insulation can degrade from rodents, environmental conditions as well as installation methods.

It's the job of the circuit breaker and fuses and a proper coordination study to ensure the protection devices operate as expected. The goal is to protect humans and equipment from these types of overcurrent conditions in electrical circuits.