Hi, i'm an electrician at a pilot hydrocarbon processing plant in Australia. I'm trying to find a rule of thumb setting for the site incoming power supply under-voltage relay time delay. The reason is that the supply to the site was interrupted which caused equipment to stop. The result of this could have very serious consequences due to the nature of the process. The relay is set to trip the incomer circuit breaker at 70% of normal voltage in a time of 0.01 secs. The time delay was set by the high voltage switch gear vendor, but i'm wondering if the time was set too short, due to the effect a trip has on the safety of the plant? My understanding of under-voltage protection is to prevent motors etc from gettig to hot because of increased current. I can't see why, if the time delay was say, 0.5 or 1 sec, the equipment wouldn't still be protected from prolonged "brownout's", but site power could be maintained if a very quick dip in power occured. I'm not sure of the duration of the dip we had, but we have two sub's with identical u/v relay's and setting's, but only one tripped.
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