What are time delay relay characteristics?

Time delay protective relays used in the power industry can be time over current or voltage, or definite time over / under voltage / over current.

Definite time type relays use an over / under voltage / current element that picks up when voltage / current reach a specified level. When this picks up, a timer is started. If current / voltage stays above / below the pickup value for long enough to allow the timer to expire, the relay trip contacts are closed.

Time over current elements use an inverse time - current curve to define speed of operation, so the more current is present, the faster the relay will operate.