Question

4. Explain Over- Current Protection Concepts in detail.

Answer 1

Overcurrent Protection and Overcurrent Protection Devices

Examples of overcurrent protection devices are many: fuses, electromechanical circuit breakers, and solid state power switches. They are utilized in every conceivable electrical system where there is the possibility of overcurrent damage. As a simple example, consider the typical industrial laboratory electrical system shown in Figure 1.1. We show a one-line diagram of the radial distribution of electrical energy, starting from the utility distribution substation, going through the industrial plant, and ending in a small laboratory personal computer. The system is said to be radial since all branch circuits, including the utility branch circuits, radiate from central tie points. There is only a single feed line for each circuit. There are other network type distribution systems for utilities, where some feed lines are paralleled. But the radial system is the most common and the simplest to protect.

Examples of overcurrent protection devices are many: fuses, electromechanical circuit breakers, and solid state power switches. They are utilized in every conceivable electrical system where there is the possibility of overcurrent damage. As a simple example, consider the typical industrial laboratory electrical system shown in Figure 1.1. We show a one-line diagram of the radial distribution of electrical energy, starting from the utility distribution substation, going through the industrial plant, and ending in a small laboratory personal computer. The system is said to be radial since all branch circuits, including the utility branch circuits, radiate from central tie points. There is only a single feed line for each circuit. There are other network type distribution systems for utilities, where some feed lines are paralleled. But the radial system is the most common and the simplest to protect.

Overcurrent protection is seen to be a series connection of cascading current-interrupting devices. Starting from the load end, we have a dual-element or slow-blow fuse at the input of the power supply to the personal computer. This fuse will open the 120 volt circuit for any large fault within the computer. The large inrush current that occurs for a very short time when the computer is first turned on is masked by the slow element within the fuse. Very large fault currents are detected and cleared by the fast element within the fuse.

Protection against excess load at the plug strip, is provided by the thermal circuit breaker within the plug strip. The thermal circuit breaker depends on differential expansion of dissimilar metals, which forces the mechanical opening of electrical contacts.

The 120 volt single-phase branch circuit, within the laboratory which supplies the plug strip, has its own branch breaker in the laboratory’s main breaker box or panel board. This branch breaker is a combination thermal and magnetic or thermal-mag breaker. It has a bi-metallic element which, when heated by an overcurrent, will trip the device. It also has a magnetic-assist winding which, by a solenoid type effect, speeds the response under heavy fault currents.

All of the branch circuits on a given phase of the laboratory’s 3-phase system join within the main breaker box and pass through the main circuit breaker of that phase, which is also a thermal magnetic unit. This main breaker is purely for back up protection. If, for any reason, a branch circuit breaker fails to interrupt overcurrents on that particular phase within the laboratory wiring, the main breaker will open a short time after the branch breaker should have opened.

Back-up is an important function in overload protection. In a purely radial system, such as the laboratory system in Figure 1.1, we can easily see the cascade action in which each overcurrent protection device backs up the devices downstream from it. If the computer power supply fuse fails to function properly, then the plug strip thermal breaker will respond, after a certain coordination delay. If it should also fail, then the branch breaker should back them both up, again after a certain coordination delay. This coordination delay is needed by the back-up device to give the primary protection device – the device which is electrically closest to the overload or fault – a chance to respond first. The coordination delay is the principal means by which a back-up system is selective in its protection.

Selectivity is the property of a protection system by which only the minimum amount of system functions are disconnected in order to alleviate an overcurrent situation. A power delivery system which is selectively protected will be far more reliable than one which is not.

For example, in the laboratory system of Figure 1.1, a short within the computer power cord should be attended to only by the thermal breaker in the plug strip. All other loads on the branch circuit, as well as the remaining loads within the laboratory, should continue to be served. Even if the breaker within the plug strip fails to respond to the fault within the computer power cord, and the branch breaker in the main breaker box, is forced into interruptive action, only that particular branch circuit is de-energized. Loads on the other branch circuits within the laboratory still continue to be served. In order for a fault within the computer power cord to cause a total blackout within the laboratory, two series-connected breakers would have to fail simultaneously – the probability of which is extremely small.

The ability of a particular overcurrent protection device to interrupt a given level of overcurrent depends on the device sensitivity. In general, all overcurrent protection devices, no matter the type or principles of operation, respond faster when the levels of overcurrent are higher.