Page 250 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 35

• These are made up using individual terminal assemblies which clip to DIN rails to make multiway strips.

• The terminal is specified in terms of the cross-sectional area of wire it will accept. This varies from 1.5 mm2
upwards.
• The most common way of terminating wires is the screw clamp with no more than two wires per terminal.

• The wire sizes are specified in data tables for plain wire ends and for those with ferrules on.
• Different colours may be used to aid identifica¬tion of groups of functions.
Contactors and relays
These are mechanical switching devices whose operation is controlled by an electromagnet. The electro-magnet
consists of a coil of wire with many turns wound on to an iron core.
When the coil of the electromagnet is energised, the core becomes magnetised and attracts a moving armature.
The armature is mechanically coupled to a set of electrical contacts. When the armature is attracted to the
electromagnet, these contacts operate and complete the circuit.
As soon as the coil is de-energised, the contacts return to normal, usually under spring. Although relays and
contactors use the same basic principle of operation, the way they achieve the end result is mechanically different.
Relays usually have a hinged armature whereas contactors usually have a stronger solenoid action, which allows
them to have larger contacts.Generally, a contactor is used to switch higher powers than a relay and needs more
current to operate.
Control relays use the same principle as contactors and look similar but are usually smaller. They are intended for
use in the control circuit and their contacts have a lower power rating than those of a contactor.

237

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