The Basics of Contactors

The Workings and Uses of Contactors

A contactor is a type of relay which is used to conduct larger currents. The contactor is used to open and close the connection to devices which require regular switching such as lighting, motors and heating. The contactor can be normally open, but closes when activated or vice versa. Contactors can be either used in AC or DC applications, but AC is more commonly used.

[A Schneider Electric four pole contactor.]

The basic design of a contactor

The contactor consists of three basic elements:

  • The spring which allows it to return to a set position.
  • The contacts which make or break the circuit.
  • The coil when energised,  is used to move the contacts into the open/closed (O/C) position.

The contactor usually has a three or four pole switch in three phase circuits or a double pole switch in single phase circuits, which in its natural position is open. The switch consists of stationary and moveable contacts usually coated with a silver alloy. The two contacts are held apart with the use of the spring. The moveable contacts are connected to an armature. A separated circuit which operates at a much lower voltage (usually 220Vac/380Vac/440Vac or 12Vdc/24Vdc) is used to power a coil wound around a magnetic core forming an electromagnet. When the electromagnet is energized it attracts the armature which then brings the two contacts together closing the circuit. The switch can either be held closed by keeping the electromagnet energized or by mechanical methods.

[A schematic representation of a simple contactor. Image created by Author.]

The contactor contacts are closed as fast as possible to avoid arcing between the two closing contacts. The arc between the two contacts needs to be extinguished as fast as possible as this could damage the contacts in the long term. The contacts are coated in silver as this prolongs the use of a contact before failure. In today’s contactors the contacts are brought together at such a speed that the contacts bounce off each other. This bouncing can cause secondary arcs which can also cause damage to the contacts. It is thus desirable for contactors to have as little bounce as possible, as contactor bouncing decreases the life expectancy of the contactor.

It is for this reason that contactors should only be purchased from a reputable authorised supplier to ensure that a premium quality product is used for your installation.

Contactor Ratings

The IEC rates contactors based on the design philosophy followed to determine the life-expectancy of the contactor. The following categories are used by the IEC to rate contactors.

It is important to follow the guidelines set out by the manufacturer when installing a contactor. Most contactors will fall into one of the listed ratings.

Contactor Applications

As can be seen in the table contactors are mostly used to operate motors. These contactors are generally of the three or four pole type due to most motors utilise three phase power. The two pole contactors are mostly used to switch lighting and heating in a building due to these systems utilizing one phase power.

For more details on contactors and their installation please contact joshb@switchman.com ,  Alternatively visit our website www.switchmanproducts.co.za

Article by: Jannes Smit, 3rd  year Electrical Engineering student at the University of the Witwatersrand. jannes9000@gmail.com

How to Select the Right Circuit Breaker for your Installation?

Selecting the correct Circuit Breaker (CB) for your distribution panel is crucial  for the longevity of the installation as well as the safety of those maintaining and occupying the premises. This article addresses the selection of key breaker attributes such as voltage, current and kA rating.

Rating Considerations:

Circuit Breaker Voltage Rating

The voltage rating of a CB is determined by the highest voltage that can be applied  across any two conductors in the circuit.  It is important to select a circuit breaker with enough voltage capacity to meet the end application. A single phase AC circuit in South Africa is generally rated at 230V  and a single pole CB rated at 230V can be used. A 3 phase AC circuit operates at 400V and requires a Triple Pole CB rated at 400V.

Circuit Breaker Current Rating

The next rating to consider is the amperage or ‘operating current’ of the breaker. CB’s are designed to operate at 100 percent of the required load . However,  in order to offset the effects of heat generated by the system, it is good practice to select a CB at approximately 125 percent of the required load.

For example: If a supply of 250A is available from the transformer, the breaker of choice for the main incomer should be rated at 250A in order to protect the transformer. However, the feeder breakers feeding a 25A load should be rated at 32A.

[Photo Taken at Switchboard Manufacturers Johannesburg]

Circuit Breaker kA Rating

Finally the ‘kA rating’ or ‘fault level’/’rupturing capacity’ of the CB should be taken into account. The kA rating of the CB indicates the maximum short circuit current that the CB can withstand without arcing or catastrophic failure. This current can be upwards of 100 times the required load and has the potential to  cause major damage to property and personnel.

For Example: A circuit breaker rated at ‘6kA’ means that the circuit breaker can withstand 6,000 amps of current during the brief time it takes to trip.

Why is it so important to choose the correct kA rating?

If the short circuit current is greater than what the CB can withstand, the contacts in the CB can weld together,  preventing it from tripping.  Another possibility is that the CB can explode, spewing dangerous plasma.

Under Rated Circuit Breaker

[Breaker fitted to a DB with an Under rated fault level]

So how do I calculate the correct kA?

The maximum current that can flow through a circuit is determined by the size of the transformer feeding the circuit as well as the length of the cable run from the transformer. This is often called the downstream short circuit current. This will determine the maximum kA rating required for the main circuit breaker.

For example: A 500kVA transformer that has a short circuit current of 35kA at its terminals. The cable run from the transformer to the main breaker is 10m and is run with 90mm2 cable. The resistance in the cable limits how much current comes from the transformer, and so after calculations it was determined that the short circuit current at the end of the cable would be 26kA. In this case, a 20kA circuit breaker cannot be used in the installation.

Switchboard Manufacturers
Distribution Panel

[Photo Taken at Switchboard Manufacturers Johannesburg]

SABS Approved Dealers:

When selecting a CB, it is vital for it to be SABS or IEC approved. This provides the assurance that the CB’s have been tested to strict quality standards and will operate in a safe manner as required. Well known brands such as ABB, Schneider and CBI are all SABS approved and are regarded as high quality devices. Switchboard Group is a registered supplier of these products and the leading manufacturer of LV panels is South Africa.

Conclusion:

In conclusion a CB should be selected based on the nominal current, kA rating, number of poles required and whether the CB is SABS approved.

Author: Brendon Swanepoel

2nd Year Electrical Engineering Student, University of the Witwatersrand

Brendon is completing Switchboard Group’s 6 week Learnership and Training program offered to students looking to further their practical skills.

Empowering South Africa’s youth.