Overcurrent protection in AC power systemsRyan Clancy | March 08, 2021
If you do not know anything about overcurrent, overcurrent protection or overcurrent protection devices, you have come to the right place! Below we examine the different categories of overcurrent, overcurrent devices and what they do, and how they work in an electrical circuit.
There are three main classifications of overcurrent: ground-fault, overload and short-circuit.
Ground-fault is known as a short-circuit condition, which is a high-magnitude fault overcurrent that places a low-resistance in parallel with the connected loads’ impedance. A ground-fault overcurrent typically only affects the grounded metal raceway/electrical distribution/utilization equipment enclosure and just one of the circuit conductors.
A ground-fault overcurrent occurs only if the electrical power distribution for the building or structure’s systems is referenced to Earth ground. Connecting the end of one or more windings of the single-phase AC transformer (wye transformer setup) to a grounding-electrode system, which creates ungrounded and grounded supply/circuit conductors, is known as “reference grounding.”
Ground-fault overcurrent magnitude is usually smaller than the short-circuit overcurrent magnitude available from the same transformer. The short-circuit can span over two (or more) single-phase transformer AC windings. Ground-fault overcurrent typically impacts just one single-phase transformer AC winding that is providing power to the faulted condition.
Overload overcurrent is quite simple; the clue is in the name. Any current that is more that the rate-load current is an overload, which occurs if an electrical circuit is directed to convey current that is greater than the load for which circuit conductors are rated. This can arise from the initial design of a circuit or by modifying an existing one.
For instance, a branch circuit with 20 A has an extra lamp added to it. This brings the load current up to 22 A in total, which results in an overload in the circuit.
An overload condition can happen in many places in a building’s electrical power-distribution system, such as at the feeder, branch-circuit level or service.
A motor can also be mechanically overloaded, and this is also classed as an electrical overload overcurrent. This can be due to excess heat, excess friction within the motor itself or any mechanical overload that occurs in the equipment that the motor drives. Overload overcurrent is typically a low magnitude control overcurrent situation.
A short-circuit current, mentioned previously, is a fault overcurrent that is high-magnitude and typically involves an unintentional connection of two or more circuit conductors. This connection across the supply and return places a short-circuit over the supply-transformer’s winding.
For example, a singular primary winding in a transformer can supply two 120 V windings (by induction) in series. A machine or some equipment will then run at 240 V when connected in the middle of the two ends of the two 120 V windings. This equipment will also run at 120 V if connected in the middle of any end of the two 120 V windings connected in series, with the remaining wire being shared by both windings.
Similarly, an AC three-phase electrical system will generally have an increased amount of short-circuit overcurrent, as the short usually involves more than one AC single-phase transformer winding.
Full overcurrent protection is only possible when the connected load and conductors can be provided with a circuit breaker or fuse at the point where the circuit begins, or where it gets the supply of power.
If an overcurrent protection device (OCPD) is installed down from the supply of power, then the overcurrent protection is separated up with ground-fault short-circuit protection along with another overload protection that is located downstream. The circuit breakers or fuse that is located downstream will give total overcurrent protection for any equipment or circuit located on the load side and giving just overload protection for the supply or line side.
What are overcurrent protection devices and how do they work?
The three main sections that make up an electrical circuit are: a load, power and a means of connecting the two.
These main components are provided with a limit control and an ON/OFF control. Both control types can control the amount of current that flows in the circuit. The ON/OFF control is typically found in switch form and can be automatic, manual, electrical or electromechanical. The limit control is generally an OCPD where electrical power is distributed and it has a circuit breaker or fuse.
The electrical power system within a structure or building has three main classifications: the feeder circuits, the service and the branch circuits.
The current rating at full-load of the connected load, the size of the load-rate of the OCPD and the conductors’ rated ampacity are all interconnected. The size of the full-load current of the connected load determines the size of the conductors for the supply side (by rated ampacity) and the settings and rating of the OCPD.
Following the same logic, the settings and rating of the OCPD along with the circuit conductors’ rated ampacity directly affect that maximum current at full load, which can be provided from the feeder, branch circuit or service. Any magnitude of current that is said to be more than the ampacity rating of the rated-load current or connecting wires of the building’s equipment, including motors, lights or transformers, is said to be an overcurrent.
The main aim of a circuit OCPD is to reduce or control the circuit conductors’ temperature to a value that will not cause damage to the conductors themselves or their insulation. This is accomplished by reducing the current that the conductors are intended to withstand. The protection of the circuit conductors from overheating has many applications and advantages, with one being the protection of the building’s electrical distribution and equipment from the results of overcurrent.
Now that you understand how overcurrent works and how to protect against various types of overcurrent, what is the best way of protecting against overcurrent? Detail your thoughts and experiences with overcurrent in the comments below!