Short circuit, overcurrent and overload protection of electrical device
All electrical equipments have their rated power. It is called overload when they surpass the rated power, and protection to the status is called overload protection. Protection to prevent internal short circuit of electrical equipment is called short circuit protection and zero-pressure is also called no-voltage protection. When there is power failure, circuit with the functions mentioned above will stop automatically and electric equipment will not start automatically when power is supplied next time. the purpose of the function is to prevent operating personnel forget cut off power source when there is power failure and electric equipment operate automatically next time when there is power and thus cause accidents. General contractor-controlled circuit has this function.
Short circuit protection
When electric appliance or wiring insulation in electrical control circuit is confronted with damages, load short circuit or wiring errors, short troubles will come into being. Transient fault current generated when there is short circuit is more than 10 times to dozens of times of the rated current. Strong electro-dynamic force of electrical equipment or distributing line due to short circuit current can damage, generate arc and even cause fire.
Short circuit protection requires to shut off power in a short time after short troubles. And common method is to connect fuse protector or low voltage circuit breaker. Action current of low voltage circuit is 1.2 times of starting current of electric motor.
Overcurrent refers to the operating state of electric motor or electrical apparatus element surpassing the rated current. Overcurrent is generally smaller than short circuit current and within 6 times of the rated current. The possibility of over current in electric current is bigger than short circuit, especially when electric motor switches on and has positive and negative inversion frequently. Under the condition of overcurrent, if current value can get right before the maximum allowable temperature rise, electrical apparatus elements can still operate normally, but impact current caused by over current will damage electric motor and the generated instant electromagnetic torque will damage mechanical transmission components. Thus, it is necessary to shut off power.
Over current protection is often realized through over current relay. Connecting coil of over current relay to circuit under protection, when current reaches to the setting value, over current relay operates. And the normally closed contact is connected to the branch where the contactor coil is to shut off contractor coil. Then, shut off the main contact of the contactor in the main circuit to shut off electric motor in time.
Overload refers to the operating state when running current of electric motor surpasses the rated current but smaller than 1.5 times of the rated current. And the running state is within the range of running state of over current. If electric motor is under the condition of overload operation for a long time, temperature rise of the winding will surpass the allowable value and be insulation ageing and damaged. Overload protection requires not operating instantly due to influence of short-time impact current of electric motor or short circuit current, and thus thermal relay is usually used as overload protection element.
When a current which is as big as 6 times of the rated current passes through thermal relay, it should wait 5s before operation. Before the thermal relay operates, heating elements of the thermal relay can have been burned out. Therefore, when using thermal relay for overload protection, short circuit protection devices such as fuse protector or low voltage circuit breaker must be installed at the same time.