Electrical Tester online
December 2021
The wrong call for contactors

The wrong call for contactors

24 December 2021

By Keith Wilson

Even in today’s electronic-dominated world, the electromechanical contactor remains almost ubiquitous in motor starting and power switching applications. It’s held this predominant position for the best part of a century and with good reason: it’s inexpensive, durable, reliable, and tolerant of abuse. As would be expected, over this extended time span, contactors have evolved significantly. This is the tale of a British company that successfully rode the first wave of evolutionary developments before coming to grief at the second.

In the early days of electric power, switchboards were exactly what the name suggests: large boards of insulating material with switches mounted on them. At first, the switches were manually operated, but before long, electromagnetically operated switches began to appear. For all practical purposes, these were the first contactors.

These early devices had a serious limitation: every component making up the contactor – the coil assembly, the switching poles, the auxiliary contacts, and so on – was mounted as a separate item, directly on the board. This made assembly difficult and modifications almost impossible. Further, if the contactor failed and caused tracking, burning, or similar damage, the whole board might need to be replaced.

To address these limitations, a French company successfully introduced a radical new development known as the bar-and-shaft contactor, also sometimes referred to as a clapper contactor. An example of a single-pole version is shown in Figure 1. With this design, the contactor itself is a complete unit rather than a kit of parts that need to be mounted individually – and accurately – on a board. In fact, these contactors were more usually mounted between steel uprights in a steel sheet enclosure, to produce a switchboard similar in overall appearance to those in use today.

Other benefits of bar-and-shaft contactors were that they offered almost limitless possibilities for customisation, so if, for example, you wanted a five-pole contactor with two of the poles normally closed, that was no problem at all. They were also easy to maintain. All of their components were simply clamped into place by screws on either the bar or the shaft and could be replaced as required. Quickly realising just how large a step forward these contactors represented, a British company, Contactor Switchgear, acquired the license to build them in the UK using component parts supplied from France.

This was in the late 1930s, and all went smoothly until the Second World War cut off the supply of French components. Contactor Switchgear responded swiftly and was soon manufacturing its own components based on slightly modified versions of the French designs. The company’s products transformed electrical control in the UK and later, via subsidiaries in Australia and South Africa, in many other parts of the British Commonwealth.

The quality of the products was unquestionable, and Contactor Switchgear supplied equipment to almost every power station and major industrial plant that was built in the UK in the immediate post-war years. Some of this equipment was still in service 50 years later.

So what went wrong? Why have you never heard of Contactor Switchgear? The answer is related to the next major development in contactors, which occurred in the early 1960s. Bar-and-shaft contactors would go on working forever, but they were bulky and expensive. For all but the most demanding applications, they were overkill. Other companies in the control gear market realised this and started to develop a much more compact form of contactor, which was dubbed “the block contactor”.

Pictures courtesy of Malcolm Palmer, Contactor Switchgear Company Archivist.

The forerunners of most of the contactors in use today, block contactors were built around plastic mouldings. They were less expensive to manufacture than their barand- shaft counterparts and far smaller. On the downside, they were much less versatile, and they were designed to be replaced rather than refurbished when they reached the end of their service life.

This was where Contactor Switchgear made the wrong call. The company’s prevailing attitude was that block contactors were an inferior product that would never catch on, except for use in the most undemanding applications. After all, they could never offer the versatility and flexibility that Contactor Switchgear’s discriminating customers had come to expect. The ‘truth’ was that very few customers actually needed the versatility and flexibility of a bar-and-shaft contactor and, for those that did, alternative solutions were almost always available.

Eventually, Contactor Switchgear had to accept the inevitable and subsequently developed a full range of block contactors. The smallest of these, the Reference 21, was rated at 5 HP and is shown in Figure 2. It doesn’t take much examination of this picture to realise that the Reference 21 was not the most elegant design in the world! There were production problems with this particular contactor, which weakened the company’s reputation, and a supposedly innovative design for a DCoperated version had issues that meant it never reached the market.

The outcome was that Contactor Switchgear lost its position as a leading innovator in the contactor market and, in 1969, it was taken over by a competitor that had ironically developed block contactors that could almost match bar-and-shaft types for their versatility. Thus ended the story of a great British pioneer.