Page 253 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Labelling

The contact arrangement and the terminal numbers are usually marked on the side of the relay, similar to that
shown here.
Two numbers are used to mark relay contacts:

• First number identifies contact positions 1,2,3, etc.;
• Second number identifies contact type. For example: 1 and 2 for NC contacts; 3 and 4 for NO contacts.
Auxiliary contacts
• Auxiliary contacts can be fitted to the top or to the sides of most contactors.

Interlocks
• Two contactors may be interlocked so that only one will operate at any time. This may be used, for instance,
when the two contactors switch a motor in different directions.
• The actual detail of fitting these interlocks differs but in general the interlock unit is fitted between the two
contactors. For exact methods of fitting, read the instruction leaflet which will accompany the contactor.

• Moving spigots on either side engage in slots on each contactor.
• If the contactors are fitted to a DIN rail they must also be clipped together using the spring clips which will be
supplied with the interlock kit.

• If they are fitted to the chassis then they are usually mounted to a plate before being fixed to the chassis.
• The contactors must not be able to move relative to each other on their mounting otherwise the interlock will
fall out.
• Once fitted, check that they will only operate one at a time by pushing down the contactor armatures.

• Further add-on parts to a contactor-type control system includes overload prevention devices.
• A protection unit may have to be fitted, e.g. a thermal overload unit.
• These have three pin connectors which engage into the contactor’s screw clamps.
• The overload unit has a changeover contact unit in addition to the three protected connections

• Most also have a clip to secure them to the base of the contactor.
Fuses
What we refer to as a fuse has several parts, the main body, the fuse holder or carrier and the fuse itself which is
called the fuselink.
There are a wide variety of types, shapes and sizes available but there are only a couple which are in common
use in control panel assembly.
• Fuses are an essential part of the safety element of the equipment.

• Because of this it is important mat the correct value and type is used as called for in the parts list.

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CITS : Power - Electrician & Wireman - Lesson 93-100