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Marsden Electrical <KeithM1229>

	circuit breakers vs fuses	205 days ago
	the phenomenon of ageing in short-circuit and overload protective devices doesn't get a lot of discussion. Schneider Electric's Richard Walley believes that it's time for this to change since, as he explains, ageing can severely compromise protection system performance: Square D brand MCBs in application A fused system. Fit it and forget it - that's what every user wants to do with protection devices in electrical installations. And, in fact, that's often what happens. Everyone forgets about the protection until a fault occurs, and then it has to work flawlessly under even the most extreme conditions. Can we, however, be sure that devices which may have been fitted years, if not decades, earlier will be up to the job? Equally, can we be sure that the devices we fit today will last? The real answer is that it all depends upon the type of protection, and the conditions to which it has been subjected since it was installed. 1) Circuit breakers: Let's look first at circuit breakers. Although these are fairly complex devices, under normal operating conditions they have no built-in ageing process. Certainly, the mechanism has a finite mechanical and electrical life. For modern breakers, however, this is measured in tens of thousands of operations and is, therefore, unlikely to be a serious consideration in any real application. In fact, IEC 60947-2 lays down high minimum standards for circuit breaker life and even these are substantially exceeded by the best available products, such as those in Compact NS range from Merlin Gerin, a brand of Schneider Electric. Under conditions of moderate overload, which is the most common type of fault, the circuit breaker once again experiences no significant ageing effect. In conventional breakers, the bi-metal elements are subjected to thermal cycling, but they are designed with this in mind, and their characteristics are not affected to any extent which could be considered significant. In breakers fitted with electronic trips, even thermal cycling is eliminated. Now, what about the acid test, a serious short circuit? According to IEC 60947, a circuit breaker must clear three faults equal to Ics, its service breaking capacity, and still remain fit for further service. In other words, neither the contact assembly, nor the characteristics of the device, must be adversely affected by these stringent tests. The breaker must also be capable of clearing one fault equal to Icu, its ultimate breaking capacity, and then be fit for use, albeit with reduced performance. The standard's use of two breaking capacities, Ics and Icu, does give rise to a potential problem. How is a user to know whether a particular fault produced a current less than Ics, or in the band between Ics and Icu? The straightforward answer is to eliminate the problem by designing breakers with Ics equal to Icu, as has been done with the Merlin Gerin NS range. The user knows for certain that a breaker with Ics equal to Icu can remain in service until it has interrupted a maximum of three short-circuit faults. This is a major benefit as, even after the most serious faults, the supply can be quickly restored as soon as the problem has been rectified. No replacement parts or spares are needed. One question remains, however: how is it possible to determine whether this is the first, second or third short circuit which the breaker has experienced? There is, of course, no readily apparent indication, but it should be borne in mind that we are considering SERIOUS short circuits. These are exceptionally rare, and when they do occur, they are sufficiently important to merit proper documentation in the maintenance records, if nowhere else. It should not be difficult, therefore, to check on any individual breaker's short-circuit history. It is also worth mentioning, perhaps, that few people easily forget either the occurrence or the results of a 50kA fault! Practical experience of circuit breakers supports this brief analysis of their long-term reliability. Schneider Electric has, for example, been manufacturing and supplying breakers for more than half a century. Long-term evaluation of these products has shown that their average life is very substantially in excess of 20 years. 2) Fuses: Now, let's turn to the main alternative to the circuit breaker, the fuse. To consider how ageing affects fuses, it's first necessary to consider the construction of a typical fuse. The important point to note is that the fuse element does not consist of the same material throughout. Instead, it incorporates a region of low-melting-point alloy. At moderate levels of overload, it is this section of the element which melts and clears the fault. It is also this section which is responsible for the most important ageing effect in fuses. There is a tendency for the metal making up the main part of the fuse element to diffuse into the low-melting-point alloy and, therefore, to change the fuse's operating characteristic. At normal operating currents, this diffusion effect is of no consequence but, under moderate fault conditions, the diffusion accelerates rapidly. This is still no problem, of course, if the fuse blows, as it then must necessarily be replaced. However, a problem does arise if a fuse element is subjected to a fault which has sufficient magnitude and duration to initiate the diffusion process, but where the pre-arcing process is not completed. This might occur, for example, where a downstream protection device clears the fault, or when a fuse blows in one of the other phases of a three-phase supply. In fact, low-level overcurrents are, by far, the most common fault on electrical systems of all types. A fuse may well, therefore, be subject to many short-term overcurrents in its life, and the deleterious effects of these occurrences are cumulative, producing the result usually described as ageing. Unfortunately, other than actually testing it to destruction, there is no way of determining whether any particular fuse has been subject to ageing. The consequence is that, to maintain the level of protection originally provided in a fuse-based electrical system, any significant overload - and certainly any short-circuit which causes a fuse to blow - should be followed by replacement of all three fuses in the feeder concerned, and also replacement of the three fuses in the immediate upstream feeder. It may also be worth mentioning that all of the replacement fuses should be sourced from the same manufacturer, since some manufacturers guarantee discrimination only with their own products. Short-circuit and overload protective devices are every electrical installation's most important insurance policy. Like insurance policies, they remain forgotten in some dark corner for many a long year, until the day arrives when, for a brief period, they assume paramount importance. You wouldn't buy insurance from a company which might not be there to deliver on that important day, so why buy electrical protection which might die of old age, just when you need it most?
	posted by Marsden Electrical