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Time Delayed Unit with Under Voltage Release

05/13/2022 10:21 AM

I understand, time delayed unit with under voltage release in MCCB/ACB is provided to not to trip for short time temporary voltage variations.

Is there any other reason to provide time delayed unit along with UV release?

If not provided, would the MCCB/ACB conflict with short circuit protection.

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Pathfinder Tags: MCCB Time Delay
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#1

Re: Time Delayed Unit with Under Voltage Release

05/13/2022 1:52 PM

Motor starting current can exceed trip current for a short time, in these cases a delay is helpful....in the case of a short circuit the current flow exceeds the delay mechanism limit and trips quickly...under voltage condition causes overheating and over time will cause a trip...

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#2
In reply to #1

Re: Time Delayed Unit with Under Voltage Release

05/13/2022 2:33 PM

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#6
In reply to #2

Re: Time Delayed Unit with Under Voltage Release

05/16/2022 4:35 AM

Um, er, those are usually used for circuit protection.

A motor overload device is something else.

Those two are usually used together and their relationship is called "co-ordination" by Electrical Designers.

BS7671 gives full protocols.

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#3

Re: Time Delayed Unit with Under Voltage Release

05/14/2022 1:18 AM

Time delay undervoltage relay protection is not normally provided on main distribution system protection except in highly specialized applications, where there is an unusual risk that may be protected for. Under voltage does not cause a circuit breaker to open.

The normal application is for large motor starters, magnetic contactors, where the contactor might momentarily open then reclose on a severe voltage dip. (Low control voltage). The undervoltage relay will keep the contactor closed for a short while, so that the motor is not disconnected from the supply, reducing transient torques and currents.

The coordination for fault protection is typically addressed by the timing difference, where dangerous currents are interrupted regardless of the voltage.

Just because a protection package offers a 27U function does not mean the designer should enable it without a clear idea of how they plan to use it.

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#4

Re: Time Delayed Unit with Under Voltage Release

05/14/2022 12:14 PM

UV release in incomer ACB/MCCB is provided when automatic tripping on loss of upstream supply is required so that the standby/Emergency incomer can be connected in shortest time.

The problem with UV release is that it may trip the incomer for a fault in one of the downstream feeders even as its own MCCB/MCB/Fuse isolates the feeder.

The short time delay (typically a minimum delay of 200ms) associated with the UV release helps in preventing such an undesirable trip of incomer.

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#5

Re: Time Delayed Unit with Under Voltage Release

05/16/2022 4:29 AM

If that is its job, then why would it be needed to do something else (rhetorical question - NNTR)?

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