Generally, the duration of an internal arcing fault test is limited to the time it takes the upstream protection to operate. Where details of the upstream protection are not known an upper limit of 500 ms can be applied to the duration of the test, but, the TR recommends 300 ms when HV protection is relied upon. (Modern protection seeing a major fault is expected to operate within this time). For a successful test the indicators must not catch fire. In addition, a number of other criteria as determined by the Arcing Class and as defined in the Technical Report must be met, e.g. minimum IP, ejected parts must not exceed a maximum weight, door and covers must remain effectively in place even if some hinges and fixings are broken.

Other considerations:
When considering the possibility of internal arcing faults within assemblies, other possibilities for increasing operator safety and/or limiting damage to an assembly may be worthwhile. These include quicker means of eliminating the arc than is possible with traditional protection and/or enhanced PPE for operators.

Arc detection
Relays are available that detect an arc within a assembly, generally through sensing a particular range of light frequencies and/or the rate-of-rise of the fault current. These relays operate within a millisecond or so. They can initiate ‘instantaneous’ tripping of an upstream breaker to isolate the fault much quicker than waiting for the normal short-circuit protection to operate. Alternatively, the operation of an arc fault mitigation device can be initiated.

Arc fault mitigation
Arc fault mitigation devices are gaining prominence. Essentially they apply a short-circuit to the supply within 3 or 4 milliseconds of the arcing fault being initiated. This reduces the arc voltage to a very low level, extinguishing the arc. At the same time, it forces a short-circuit operation of the upstream protection. This has the negative effect of stressing the upstream network and forcing a maximum level operation of the protection. However, the very fast operation of the fault mitigation device has several benefits; It is worth noting that arc fault detection and arc fault mitigation devices must be operational and maintained in order to provide effective protection.

Working on or near energised equipment:
As previously noted, one of the most likely causes of an internal arcing fault is an ‘incident’ while maintaining or modifying a partially energised assembly; a tool might slip, something might drop through a ventilation aperture in a barrier, a conductor might be presumed dead when it is energised, etc. These types of activities are outside the scope of IEC/TR 61641, but if they are contemplated other safety and legal obligations must be met.

Network topology:
In recent years there has been a trend towards the use of larger transformers or more transformers operated in parallel. As current rating of assemblies increases, so in general do the short circuit currents and the energy released under arcing Subject to the many other conflicting needs, if at the design stage the magnitude and duration of the prospective short circuit can be reduced by, for example, using more and smaller transformers, wider use of current limiting protection, the effects of an internal arcing fault within an assembly can be correspondingly reduced.

Forms of separation:
Forms of separation in accordance with IEC 61439-2 are intended to improve access to parts of an assembly while other parts remain energised. They might also assist in containing an arc to a compartment within an assembly, but this is not their intended function. They are not verified as having any arc containment capabilities.

Decision:
Making the ‘right’ decision in respect of an assembly being arc fault tested is not easy; it is subjective, and confirmation that the right decision has been made might never be known. Anyone specifying an assembly has to decide if the additional benefit and cost of an internal arc fault tested assembly is justified. This can only be objectively established for each individual application, taking into account when internal arcing faults are likely to occur and if testing in accordance with IEC/TR 61641 would avoid any issue that may result. Anecdotal evidence is that, in the case of assemblies in locations with controlled access, the majority of arcing faults which result in injury occur when the assembly is partly energised and some form of work is being undertaken inside the assembly. At other times the assemblies are unattended. When deciding if it is worthwhile specifying an arc fault tested assembly all the aspects given in Table 1 should be reviewed. Whilst most items to be considered are not definitive, an objective review of each of them will assist in reaching the most appropriate conclusion for the application. (As significant judgement is required, some form of scoring or ranking system can assist in reaching a conclusion.

Specification:
Working on, or inside, an assembly while it is partially energised is outside the scope of IEC/TR 61641. If activities of this nature are being considered the appropriate legal obligations and safety practises should be followed. Installing arc detection and arc fault mitigation devices can dramatically reduce the consequences of an internal arcing fault in respect of personnel safety, damage to the assembly and time to return the assembly to service. The cost of these systems must be weighed against the probability and consequences of an arcing fault and the impact their operation may have on the upstream system. Specifying an assembly to be internal arc fault tested in accordance with IEC/TR 61641 is beneficial in some circumstances, but making the right decision for the application and ensuring the desired benefits are realised is very difficult. The benefits are subjective and depend on the application, probabilities, consequences for personnel safety and operational impacts. Claims of meeting IEC/TR 61641 must be thoroughly reviewed with respect to the extent and validity of testing performed. There is a cost associated with providing an assembly in accordance with any of the Arcing Classes given in IEC/TR61641. The key question is; has the risk of an internal arcing fault within an assembly and the consequences of that fault been reduced to an acceptable level? Determining if an assembly should have an Arc Classification and which is the most appropriate Class needs serious consideration. Aspects to be considered and guidance on reaching the ‘right’ decision is given in Table below.