Page 255 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

• Latching - sometimes called on-off or, with a button actuator, push on/push off where the contacts lock in one
position when the button is pressed then released and only change back when the button is pressed a second
time. Stay-put is yet another name.

Rotary actuators can provide more than two positions and may be used to provide a selector-type switch.
Switch adaptors
These are used to hold the contact elements. Standard adaptors hold up to three contact elements alongside
each other. Some adaptors are made complete with contacts (contact blocks).
• Front-mounting contact block. This clips to the actuator. The contact elements then clip into the rear of the
adaptor.
Switch contacts

There are two basic types of contact:
• Normally open (NO).
• Normally closed (NC).
• A changeover set (CO), can be made from a combination of one NO and one NC by wiring them together as
shown.
Rotary switch diagrams
The contact diagrams for rotary switches are often accompanied by an operational grid showing which contact
operates in each position.
• Front panel view of a 4-position rotary switch with an ‘off position.
• This is sometimes referred to as a 0 position, 1 pole, 3 step switch.
• This is the circuit diagram showing the individual switch elements, in this case all NO.Lamps

Two symbols are shown recommended by BIS. • Indicator lamp.
• Signal lamp.
It is not important from an assembly point of view which is which.
The majority of indicators are panel-mounted. These consist of two main parts, the lamp holder and the bulb. The
lampholder can take several forms:
• One-piece holder which fits through a hole in the panel.
• A nut holds it tight to the panel. Take care not to overtighten this otherwise the holder may be damaged.
• The bulb is fitted from the front.

• This type is very similar and fits in the same way but the bulb is fitted from the rear.
Earthing - the protective bonding circuit
The earthing of electrical equipment is a protective measure designed to protect us from electric shock by
preventing the exposed conductive parts of the equipment from becoming live should a fault occur.

The exposed conductive parts are things like the metal cabinet housing the control circuits, the metal trunking
carrying cables from the cabinet to the machine and the machine itself. Technically, they are all the conductive
parts of the system that, under normal conditions, are not required to carry electric current
To see why this system works, we have to look at how the electricity supply is connected into a factory building.
The electricity is generated at the power station and then fed at very high voltages through the national grid.
Eventually it will arrive at the local sub-station where it will be connected through a transformer to supply the
factory.
Control panel layout and wiring practise

The quality of the wiring methods used in an industrial control panel can vary quite widely. This lesson summarizes
some best practice when it comes to control panel layout and wiring.

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