Underground » Detection and Prevention of Fires and Explosions
An investigation has been conducted to examine the safety aspects of IP55 sheet metal enclosures used for housing switchgear in underground coal mines. The study examined the possibility of venting these enclosures to minimise the hazards to personnel from enclosure bursting and from the explusion of hot gases and molten material during high power arcing fault.
Method
Tests were carried out using a high pressure air reservoir to predict the variation of pressure rise with vent area and examine the strength of two IP55 enclosures (one 0.92m3 stainless steel, the other a three compartment mild steel enclosure in which a 0.36m3 compartment was used).
Arc fault tests were conducted at 6 and 9kA using a 3.3kV supply voltage, on two flameproof enclosures of 0.3m3 and 0.84m3 fitted with vents ranging in area from 0.02m2 to 0.17m2, and on the two IP55 enclosures. A range of flame retarding materials for use in the venting path was investigated for effectiveness in cooling gases and molten material expelled from the vents.
A computer model was developed to simulate the pressure rise for various enclosure volumes and power systems fault level.
Comparisons of results from modelling, air receiver and arcing tests show that the air receiver tests in combination with computer simulations may be use to predict the variation in pressure rise with vent area for various arcing conditions and verify the strength of an enclosure (to a level predicted for internal arcing), for cost effective initial prototype testing of IP55 underground mining enclosures.
A method of comparing qualitatively the effectiveness of flame retarding materials used in a vent has been developed, and used to identify a number of combinations of materials which could be used in a vent to reduce significantly the risk of arc products causing coal mine fires. The effective combinations of materials included a robust grill designed to trap large molten particles, and material with high surface area and thermal conductivity such as stainless steel foam or a number of stainless meshes to cool the gases.
Results
Issues which affect the design of sheet metal enclosures have been addressed. In particular it was found that:
- door fasteners are often the weakest points in enclosure design - failure of these during are faults can put operators at risk;
- small enclosures especially need vent mechanisms since they are subjected to high internal pressure rises in the event of an high power arcing fault;
- large enclosures may not require venting but if they do, they will require larger vents than small enclosures to keep the maximum pressure rise down to a particular level;
- vents must be appropriately sized for the enclosure volume and fault level;
- vents bursting mechanisms, required to maintain the IP55 rating of the enclosures must release at low levels of enclosure internal pressure rise. If no, the vent either will not operate (and the enclosure may burst) or the vent will burst but the enclosure will be subjected to higher internal pressure rises than would have otherwise been experienced with low pressure vent relief.
Recommendations
It is recommended that mine operators conduct safety audits of switchgear enclosures.
It is recommended that manufacturers take steps towards improving the safety of sheet metal switchgear enclosures by reducing the potential for arcing faults, limiting the duration of arcing faults, improving the design of enclosures and vents and that provisions for arc fault testing of sheet metal switchgear enclosures be included in relevant mining standards.