Predicting short circuits

Arc detection systems, are much more practical for fitting within new build switchgear, proving costly and technically difficult (if not impossible) to retrofit.

Optical systems rely on light transducers being situated at likely arc flash points within switchgear. Their attractive feature is the speed of operation once an arc is detected, usually a few milliseconds. However, it is still a reactive function, a post-fault event - damage limitation rather than preventative detection. In order for the system to work the fault current must mature to an explosive release of energy such that an arc is created.

Such systems are limited to visibility of arcs within the switchgear itself. It cannot consider any arcs beyond the switchgear boundary, within cable runs or peripheral equipment.

The cable may snake through junction boxes, underground, along cable trays, through bulkheads to terminate at a motor. If there were ever a networked ignition source, it would surely be the cable carrying a fault current. If that fault current resulted in an arc, its location would be unpredictable and potentially catastrophic.

Present arc detection systems rely on the sequence of high fault current, creation of arc, rapid detection, trip. Arcing does not necessarily mean a stereotypical phase-to-earth or phase-to-phase flashover electrical fault. There is a more prevalent condition called series arcing, where a conductor develops an in-phase arc, resulting from physical damage or prior heat build-up caused by fatigue or at a loose or intermittent connection.

Here, the current is limited by the load current of the equipment. The series arc can develop as a hot spot of higher resistance. As it deteriorates, it may start to spark, yet this can continue undetected and connected equipment could appear to operate normally. Only after the insulation has broken down sufficiently would the series arc develop into a detectable short circuit type arc.

Other indicators

However, there are other early indicators of short circuit current that can be measured and detected using nothing more than the existing current transformers (CTs). This can be used to identify the high resistance joint and of the intermittent sparking connection. Detecting these conditions, as a likely precursor to short circuit, can be done so faster and also at a much lower current level. It is therefore possible to provide protection to the system before any serious equipment damage occurs.

This is not a new idea - in 1980, Electrical Review published an article concerning sensitive short circuit protection for induction motors in mining applications. It recognised that if the cable is too long and the impedance too great, then a short circuit fault at the motor end would not be detected by conventional short protection at the switchgear. Therefore there was a desire to provide sensitive short circuit which would allow cable runs to be longer before switchgear underground needed to be moved.

The Motor Arc Protection (MAP) relay is based upon the idea that detection needn’t require high magnitude fault current in order to trigger short circuit protection. Using conventional CTs and VTs to measure current and voltage, it can offer arc protection along a cable and to a motor through the analysis of phase angle movement.

By utilising existing conventional CTs and VTs, without additional transducers, MAP can be easily retrofitted to existing circuits. It provides a method of protection which can respond to a developing arc or resistance condition within the motor or cable. This would greatly enhance the protection of motors and in particularly for Mining, Oil, Gas and Petrochemical environments.