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Refuse That Fuse!

A large percentage of conventional motor starters still rely on fuses for short circuit protection. That's surprising, says Steve Rickard of Moeller Electric, because, as he explains, fuseless starting solutions have so much more to offer.

A large percentage of conventional motor starters still rely on fuses for short circuit protection. That's surprising, says Steve Rickard of Moeller Electric, because, as he explains, fuseless starting solutions have so much more to offer.

You've heard the arguments about fuseless starters saving space, using fewer components and needing less wiring. You also know that it's easier to reset a breaker after a fault than it is to dig out a set of replacement fuses – even assuming that suitable spares are to hand – and fit them. Yet still you're not convinced that fuseless starters are a good idea.

Let's try to find out why, first by dealing with some of the main concerns that are raised in connection with fuseless starting solutions, and then by looking at some of their less obvious, but nevertheless significant benefits.

Before we start, it's probably a good idea to decide exactly what we mean by a fuseless starter. It's typically a device which combines, in a single unit, overload and short circuit protection together with some method of switching the motor on and off. In their basic form, most fuseless starters are manually operated.

For this reason, fuseless starters are sometimes seen as inflexible, since additional components are needed for automatic control. But let's take a look at what's really involved. The best of modern fuseless starters can be fitted with remote actuators to switch them on and off which simply clip into place.

The actuators are invariably available in versions which are suitable for use with conventional relay-based control systems or for direct operation from low-power PLC outputs. And, they have a big benefit over contactors: they consume power only during switching operations. The rest of the time, they can simply be left unpowered, cutting energy consumption and heat generation within the panel.

It's true that remote operators are not suitable for frequent switching, but again it's important to get this in perspective. Modern types can handle up to 60 switching operations per hour, and have a life of around 50,000 operations. In the vast majority of cases, this is well in excess of what's needed.

For those applications which really are more demanding, the best suppliers now offer add-on contact blocks for their fuseless starters. These work in a similar way to a contactor, but have the advantage of mounting directly on the starter, saving space and wiring time. In addition, they are designed to match the starter's electrical characteristics, so that issues like co-ordination are never a concern.

So much for ordinary direct-on starters, but what about reversing and star-delta types? Once again contact blocks, used in conjunction with mechanical interlock devices, provide an easy and cost-effective solution.

Now let's turn to breaking capacity. Many designers still believe that fuseless starters need fuses to provide back-up protection in applications where high fault currents are possible. This may once have been true, but it certainly isn't any longer.

Because of the high impedance of their overload protection devices, small fuseless starters are inherently short-circuit proof. Larger types generally have good short-circuit capacity but, where necessary, this can be further increased by using a current limiting module.

Easily fitted to the starters, these modules basically comprise a set of spring-loaded contacts. The contacts are specially designed so that when a high fault current flows, the magnetic forces generated drive them into the open position. This effectively limits the fault current and gives the protective device time to operate.

When the fault current ceases, the contacts in the current limiting module automatically reset to their normal state. The modules therefore provide all of the back-up protection necessary without the inconvenience of having to use bulky fuses which always need replacing after a fault.

Before leaving the concerns which are raised about fuseless starters, let's look at an item which always looms large – cost. It's generally believed that fuseless starters cost more than their fused counterparts. And sometimes they do – if you look only at the component costs. But take into account savings in wiring time together with the reductions in control panel space requirements which make it possible to use smaller, less costly enclosures, and it soon becomes apparent that fuseless starters can actually save money.

Lifetime costs If lifetime costs are considered, the arguments in favour of fuseless starters become even more convincing. For a start, there will never be any fuses to replace and, because resetting the starter after a trip takes just a second or two, costly plant downtime is kept to a minimum.

We've already mentioned many of the more obvious advantages of fuseless starters, but it's also worth taking a look at some of the less often mentioned benefits, as these can be equally important.

The first is simple – since all fuseless starters effectively incorporate an on-load switch to IEC/EN 60204 & IEC/EN 60947, the need for a separate isolator is eliminated. This saves space, money and wiring time. Modern products also incorporate separate signalling contacts for overload and short circuit trips. This speeds fault diagnosis, particularly when automated diagnostic systems are in use, thereby helping to reduce downtime.

Some types of fuseless starter also feature interchangeable plug-in trip modules, allowing the starter to be fitted in a control panel and fully wired even if the final rating of the motor is not accurately known. This is a particular benefit when motor sizes have to be selected or changed during commissioning.

A further advantage is that removing the trip module renders the starter totally inoperative, just like removing the fuses in a conventional starter. This can be a valuable safety feature during commissioning and fault finding.

There can also be important benefits for specialised applications, such as those involving the operation of motors in hazardous areas. Fuseless starters are now available which are suitable for the protection of EEx motors, and which are covered by ATEX approvals. These devices offer a much more convenient and straightforward approach to building ATEX compliant control systems than using fused starters assembled from separate components.

Finally, fuseless starters solve many of the problems associated with producing equipment for export. Around the world, many different and incompatible types of fuses are used. This means that, in equipment where fused starters are fitted, different versions must be produced to satisfy the local requirements of each market. In contrast, with fuseless starters, a single model is all that's needed.

When all of the benefits of fuseless starters are considered together, it's very difficult to come up with sound technical or commercial reasons for the continued widespread use of starters which rely on fuses for short-circuit protection.

Inertia – "we've always used fuses" – seems to be part of the answer, but it's not really a factor which ought to be influencing design decisions in the 21^st century. So, if you're still using fused starters, it might just be time to consider switching to a modern starting solution that has rather more to offer!

This page last updated: 1 April 2008