Short Circuit with Time Delay

Dear Sir,

The circuit breaker has closing and latching capability, and interupting capability. When closing and latching capability meet the calculated current, but interrupting capability is not enough, let say the interrupting duty is 5 cycle but the calculated current until 5 cycle still higher than interrupting rating CB, a time delay is required.

May be this information can help you.

Regards,

Time delay circuit interrupting devices are usually used in motor control circuits due to high inrush current on start up.

Please disobey this comment if it is out of the track.

My comments is related with GE-IPSEO study for 34.5 kV ring bus system implementation for LNG Plant in Sep 9, 1983, by J.R Dunki Jacob.

As a result of s.c. study, they found that 500 MVA class which have interruting rating of 37 kA sym. base on the calculated x/r ratio. To meet the interrupting duty magnitude, it was necessary to delay the interrupting time by 2 cycles. As a concequence, the interrupting duty (37 kA as a result computer calculated) of 5 cycles breaker has to be reduced to reflect the additional 2 cycles interrupting time. The end result of the calculation by adding 2 cycles timer is 19.8 kA. This condition is only temporary before upgrading these circuit breakers.

additional.

Time delay for inrush current during motor start-up. I have found this situation in the first time using microprocessor relay on cooling water pump motors (4415 Hp, 13.2 kV, 3 phase, 50 hz) never succed from start-up before.

Our investigation revealed that the actual inrush current is 11 times of motor FLA, and relay setting from manufactur is 1 cycle. Using electro-mechanical relay this current cannot be detected, but for MPR relay it was sensed.

Our action is to change the delay time from 1 cycle to 3 cycles.

A time delay in the short-circuit release of a circuit breaker is usually needed to provide discrimination between upstream and downstream breakers. Please check out this link...

www.schneider-electric.com/documents/.../en/.../electrical.../ect167.pdf

I understand that your question pertains to the protective release options in circuit breakers.

When a short circuit happens in the network, only the breaker closest to the fault should trip first. Just in case this breaker doesn't trip, the next upstream breaker should trip, but only after sufficent time delay, duly waiting for the downstream breaker to respond first. So, obviously, the short circuit releases in breakers should have a time delay setting to enable this time based discrimination.

But, if the short circuit current value is much higher than the short time withstand rating of the breaker, the release must be set to instantaneous.

To put it in a nutshell, short circuit with time delay is necessary to achieve time based discrimination in a radial fed system and short ciruit instantaneous is necessary to protect the breaker from exceeding its short time withtstand ratinng.

In some circuits,the tripping action for the short cct breaker is too is too frequent and uncalled for.It does not allow for normal or smooth operation of a system,sort of nonsense tripping,As a result the tripping action can be reset or adjusted from the time settings on the breaker to suit yr normal trip time with a little delay by increasing the time say for example,from 1s to 2or3s.

Patrick Whowha