This assembly constitutes what is called "the power circuit" and provides three essential functions which are:
• the isolation function allows the separation of the network circuits and the lockout, it is performed by the disconnector;
• the control function allows the opening and closing of the circuit , it is performed by the contactor;
• the protection function provides protection against overloads and short-circuits, it is performed respectively by the thermal relay and by the fuses.
Notes: the fuses are generally carried by the disconnector.
The fuses
Fuses, like circuit breakers that we will see a little later, are equipment protection devices. The use of the HV/LV Switchgear comes important there.
Cartridge fuses and fuse holder mounted on rails
They are used to open the circuit in the event of a short circuit, thus protecting the motor and switchgear line.
The fuse is the weakest element in the circuit; it is equivalent to the shear pin in mechanical equipment.
It is designed to open the circuit when the current flowing through it is greater than the rated current of the fuse. This is because when the current flowing through the fuse increases, the heat rises. The ratio between current and heat is:
• Heat = time x (current) (current)
• So if the current quadruples, the heat will be multiplied by a factor of 16.
This excess heat melts the conductive element, which opens the circuit. It is important to know that when a fuse "blows" it could be extremely hot. Therefore, when dealing with large fuses, use tools (following figure) so as not to touch the fuse directly.
Fuse clamp:
Types of fuse
There is a wide variety of fuses. Figure 4.26 shows the main families you will encounter in the industry.
There are three types of cartridge fuses depending on their use:
• The gF (domestic use) and gG (formerly gI and industrial use) cartridges are intended for the protection of distribution lines. They are marked in black.
• AM cartridges (engine support) are designed to withstand certain overloads such as starting an engine. They are marked in green.
There are two types of cartridge fuses depending on their rating:
• Cylindrical cartridges, in eight sizes, used for currents from 1 to 125 A.
• Cutter cartridges, in fifteen sizes, used for currents from 16 to 1250 A (only in gG and aM).
We also find for these types of fuses:
- three voltage classes: 250 V, 380 V and 500 V;
- with or without indicator light indicating the state of the fuse;
- with or without striker allowing to act on a microswitch (opening of a circuit, signaling, etc.).
The last type of fuses you will see are those used in electrical appliances.
They are generally small in size, hence their name miniature fuse. You can see an example in the following figure. They have templates ranging from 0 to 60 A.
Fuse families:
Fuse installation
Important points to remember when replacing a fuse are:
- always replace a fuse with another of the same rating (same rated current) and of the same type;
- if a fuse has been replaced several times in a short period of time , the cause of the fault must be sought.
About the Company:-
Robert Cowlishaw and Associates was established in the year 2007 and is a specialist Electrical Engineering company situated in Stockport near Manchester in the UK.

We provide Electrical Survey and Condition reports including financial reports on various types of buildings, mainly commercial and industrial buildings.