Electrical networks and machines are subject to various types of faults while in operation. Short circuit and overload are the most dangerous faults in the power system. They change the circuit structure, causing changes in power distribution, which will bring energy loss, and damage to the stability of the power system, affecting the normal operation of electrical equipment. Despite their similarities, these two phenomena are different from each other.
The differences between electrical short circuit and overload are the following:
The main difference between short circuit and overload is the definition. An overload is a condition at which an electrical load draws a current above its rated value, for a prolonged period. A short circuit is an accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these conductive parts to be equal or close to zero.
Other definitions of short circuit:
- An unwanted low-resistance connection between two points in an electric circuit.
- An abnormal electric path.
- The condition occurs when a circuit path is created between the positive and negative poles of a battery, power supply, or circuit.
- A situation occurs when hot and neutral wires come in contact with each other.
- An accidentally established connection between two points in an electric circuit, such as when a tree limb or an animal bridges the gap between two conductors.
Other definition of overload:
- Electrical overload is a type of overcurrent where more amperage is put across an electrical wire or circuit than it can handle leading to excessive generation of heat, and the risk of fire or damage to equipment.
2. Voltage levels
A short circuit has zero voltage across it for any finite current flow through it. On a circuit diagram, a short circuit is designated by an ideal conducting wire with zero resistance. In an overload, the voltage can be low, but cannot be zero.
3. Current levels
The maximum value of the short circuit current is directly related to the size and capacity of the power source and is independent of the load current of the circuit protected by the protective device. The larger the capacity of the power source, the greater the short circuit current will be. Overload current is directly related to the capacity of the electrical load. That’s why the current level of the short circuit is much higher than the overload current. The short circuit is multiple of the rated current. Overload is close to the rated current.
4. Danger levels
A short circuit is more dangerous than an overload. Because the current level is higher. A short circuit is extremely dangerous in high-voltage applications. A short circuit has fast energy transfer but an overload has slow energy transfer.
Electric power systems are designed to be as free of short circuits and overloads as possible through careful system and equipment design, as well as proper installation and maintenance. However, even with these precautions, short circuits and overloads do occur.
Some causes of short circuits are:
- Presence of vermin rodents in the equipment.
- Loose connections.
- Voltage surges.
- Deterioration of insulation.
- Accumulation of moisture, dust, concrete juice, and contaminants.
- The intrusion of metallic or conducting objects such as fish tape, tools, jackhammers, or payloaders.
Some causes of overloads are:
- Excessive consumer loads.
- Faulty appliances.
- Bad wiring and grounding.
- Improper use.
6. Protection devices
Short circuit and overload must be quickly removed from the power system and this is the job of the circuit protective devices. To accomplish this, the protective device must have the ability to interrupt the maximum current that can flow at the device location.
Fuses and circuit breakers can protect systems against overload and short circuits. Thermal overload relays only protect against the overload. Magnetic only breakers only protect against only short circuits.
When a short circuit occurs on a power system, several things happen – all of them are bad:
- At the short circuit location, arcing and burning can occur.
- Short circuit current flows from various sources to the short circuit location.
- All components carrying short circuit currents are subject to thermal and mechanical stress.
- System voltage drops in proportion to the magnitude of the short-circuit current.
The consequences of overload are:
- Increase of the electrical energy consumption due to residual current caused by isolation issues in the wiring.
- Personal accident by direct contact with damaged wires.
- Sustained overload can result in a short circuit.
The calculation of short circuit currents is essential to the selection of adequately rated protective devices and equipment in industrial and commercial power systems. Today, power systems carry larger blocks of power and have greater safety and reliability requirements. There are lots of parameters that should be considered when calculating short circuits. For this reason, the calculation of short circuits is not easy and must be done carefully.
9. The reaction of protection devices
The reaction of the protection devices against short circuits is extremely fast. But the opening times of the overload protection devices are delayed. Fast tripping is vital in short-circuit protection.
Overloads cause thermal tripping but short circuits cause electromagnetic tripping in the protective devices.
Overloads shall be switched off only when the heat produced by them passes a pre-determined limit. For reason, the tripping time for smaller overloads is correspondingly longer than for heavier overloads. Short circuit currents shall be broken without any delay.
There are four basic sources of short circuit current:
- Synchronous motors
- Induction motors
- Electric utility systems
All of these can feed a short circuit current into a short circuit.
The source of an overload can be any electrical load.