Technical Bulletin
Be sure arc faults are not your fault
Byline goes here
Switchgear related accidents do not happen often but when they do, they are likely to be severe or fatal.
So while human error cannot ever be excluded, manufacturers should ensure they build the highest level of safety precautions into their switchgear.
It has always been common practice to include arc fault containment in medium voltage (MV) and high voltage (HV) switchgear. Current is not different on the low voltage (LV) end and, in fact, might even be higher there. Therefore, global IEC standards and Australia Standard 3439 recommend arc testing.
However, a safety standard designed to empower users of LV switchgear to insist manufacturers prove their equipment prevents injury from internal arc faults has sat largely idle since 1996.
But arc faults are not idle and do happen. They can never be totally predicted or prevented, and are usually caused by factors outside the control of the manufacturer. What can be controlled, though, is the extent of the damage and injury caused by an arc.
The IEC 1641 standard marks the first attempt to measure the ability of LV switchgear to contain the effects of an arc. It is a set of guidelines that can be requested by customers requiring an assurance the switchgear assembly specified provides a basic level of safety.
As such, IEC 1641 could help substantially reduce the likelihood of serious injury and damage.
Unfortunately, with its requirement for destructive testing, testing to IEC 1641 can add as much as $150,000 to the price of a switchgear assembly. This, combined with a common belief that switchgear with Form 4 separation type tested to BS EN 60/439-1:1994 offers sufficient protection against arcing, means demand for the test has so far been limited.
To date there have been only 30 tests performed at ASTA approved laboratories.
This has led many to question the purpose of IEC 1641 and the value of conducting testing. To answer this, it is first necessary to explore the potential consequences of an arc fault, and to discover how its cost can far exceed that of a one-off test to IEC 1641.
What is arcing?
An arc fault normally begins as a short circuit between two or more contact points, and can quickly develop to pose a serious threat to personnel safety.
With an internal core temperature of 20,000°C, an arc lasting just 500 milliseconds can cause tremendous damage not just to a switchgear assembly, but also to any other object, including a human being, within its proximity.
Arc fault testing is a contentious issue; not least because internal arc faults are most often as a result of human error or negligence. And there has been reluctance to accept that arcing tests could be devised that would be reproducible and meaningfully representative of what might occur in practice.
The behaviour of an arc and its secondary effects may be impossible to predict. Consequently, it has not been possible to arrive at an internationally acceptable standard for tests that would take into account all the possible ramifications of internal arcing faults.
The absence of such a standard means the ability of a manufacturer’s assembly to withstand the effects of an internal arc fault may never be known until one actually occurs.
With damage caused by an arc potentially amounting to thousands of dollars, not to mention the cost of lost productivity, switchgear customers should demand some extra form of testing.
It is perhaps easier to say what IEC 1641 is not. It is not a type test or a compulsory test. Nor is it an attempt at a catch-all definition of how arcing occurs and how to prevent it.
Furthermore, the results of the tests are not for presentation in the form of a test certificate, since judgement as to the effectiveness of the protection achieved is solely a matter for agreement between the manufacturer and the customer, and depends greatly on the application in question.
Although many LV switchgear manufacturers were carrying out their own safety testing as long ago as 1984, there were no guides as to the conditions under which this testing had to be conducted. Consequently, a customer had no way of comparing the safety of one manufacturer’s switchgear with another’s.
Common guidelines
The introduction of IEC 1641 has given manufacturers common guidelines for carrying out certain tests. Derived from a German standard, VDE 0660 part 508, IEC 1641 provides a number of criteria aimed at assessing the performance of a LV assembly in the event of an internal arc fault.
According to IEC 1641, an arcing test must be performed at a voltage at least equal to the highest rated voltage of the assembly. The points within the assembly at which the arcs are initiated then have to be chosen so that the effects of the resulting arcs produce the highest stresses. Usually, these points could be on the incoming and outgoing sides of a short-circuit protective device and across the busbars.
To pass the test, the switchgear, which must have been tested with all doors and flaps closed, must satisfy any of five criteria selected by the customer (see box).
This criteria states structural integrity must have been maintained, with all doors, flaps and windows remaining closed and attached and that the likelihood of injury to a person working in the proximity of the board caused by flames or hot gases is minimised.
Critics of the test question its value as a true measure of safety in the event of an internal arc fault, citing the failure of arc fault tests such as those in the Australian standard AS 1136 Appendix EE to consistently achieve the same results. They argue that little value should be attached to any arc safety test, since the mysterious behaviour of an arc makes exact repetition or reproduction of an arc impossible.
But IEC 1641 is not concerned with the level of internal damage to an assembly or to what extent the spread of damage to other compartments within that assembly may be prevented. Instead, it is a set of guidelines aimed at assessing the ability of an assembly to limit the risk of injury to people in the event of an internal arcing fault.
Test or not to test
There are several reasons why customers should choose safety over cost when deciding whether to opt for an assembly tested to IEC 1641.
Firstly, there is the fact that even switchgear manufactured to BS EN 60/439-1:1994 will not provide total protection against arcing. Of the manufacturers that have so far carried out testing at ASTA approved laboratories, all of which tested equipment meeting the standard, 25% failed to pass one or more of the tests set out by IEC 1641.
Secondly, and perhaps most importantly, selecting those manufacturers capable of meeting IEC 1641 will provide an important assurance of quality that not all manufacturers can meet.
By undertaking a continuous test program, including tests such as IEC 1641 conscientious manufacturers are showing a commitment to not only the life of their products, but their customers.
Negligence by ignorance is no longer an acceptable defence in the eyes of the law, therefore, every specifier and potential customer of LV equipment assemblies should be striving to use the best available. And in doing so they should always put their suppliers to the test.
Regina Roos is ABB sales manager LV Systems Australia and NZ.
23 July 2003