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Refuse that fuse!01 September 2008A large percentage of conventional motor starters still rely on fuses for short circuit protection. That’s surprising, says Steve Rickard, as fuse-less starting systems have so much more to offerYou’ve heard the arguments about fuse-less 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 fuse-less 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 fuse-less starting arrangements, and then by looking at some of their less obvious, but nevertheless significant benefits.
A fuse-less starter is typically a device that 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 fuse-less starters are manually operated. It is probably for this reason, fuse-less 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 fuse-less starters can be fitted with remote actuators that simply clip into place to switch them on and off.
Actuators are available in versions that are suitable for use with conventional relay-based control systems or for direct operation via low-power PLC outputs. And, they have one big benefit over contactors - they consume power only during switching operations. The rest of the time, they can simply be left powered down, 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 that really are more demanding, the best suppliers now offer add-on contact blocks for their fuse-less 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-online 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.
Turning to the subject of breaking capacity, many designers still believe that fuse-less starters need fuses to provide back-up protection in applications where high fault currents are likely. This may once have been true, but it certainly isn’t any longer. Because of the high impedance of their overload protection devices, small fuse-less 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, specially designed so that when a high fault current flows, the generated magnetic forces 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. So all the back-up protection is provided without the inconvenience of having to use bulky fuse, which always need replacing after a fault.
Setting aside the operational concerns about fuse-less starters, there’s the question of cost. It is generally believed that fuse-less 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 that allow the use of smaller, less costly enclosures, and it soon becomes apparent that fuse-less starters can actually save money.
If lifetime costs are considered, the arguments in favour of fuse-less 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.
While the more obvious advantages of fuse-less starters have been outlined above, it is also worth noting some other benefits. For example, since all fuse-less 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.
Some types of fuse-less 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. Fuse-less 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, fuse-less 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 fuse-less starters, a single model is all that’s needed.
When all of the benefits of fuse-less 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” – is often put forward, but this shouldn’t influencing design decisions in the 21st Century. So, if you’re still using fused starters, it might just be time to consider switching to a modern motor starting system that has rather more to offer!
Steve Rickard is with Moeller Electric Contact Details and Archive...Related Articles...Most Viewed Articles... |
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