Relay Instantaneous setting

[qrenzo]

Helo, I'm a newbie studying about power system protection. My question is:

http://www.hv-eng.com/OvercurrentCoord.pdf


In the above link, at page 52 at the "Coordination Pop Quiz #3" slide, the author states that:
"A main should never include an instantaneous setting"

I think he meant that for a main CB, you shouldn't include the instantaneous setting.
Can anyone explain why and what is the basis for this particular recommendation?
( Please give your reference / standard )

 

[Sahib]

If the main breaker is equpped with intantaneous setting, it is also liable to trip along with any other downstream breaker feeding any shortcircuit thereby losing the selective coordination.

 

[jdsmith]

The assertion that "a main should never include an instantaneous setting" is true for classical low votlage and medium voltage selective coordination. In the modern low voltage world there are numerous ways to include instantaneous settings on upstream devices that are fully selective with downstream devices. In the medium voltage world there is also an increased emphasis on applying high speed protection throughout the system, although it is usually not an instantaneous on a main for coordination reasons. Bus differential, transformer differential, and zone selective interlocking (ZSI) are often used to provide high speed MV protection.

Keep in mind that the 2011 NEC 240.87 requires an instantaneous setting or other equivalent high speed protection on low voltage protective devices. The code making panel's intent here was to ensure that personnel working on the affected systems are protected by high speed protection. Under the 2011 code there are some ways to weasel around this requirement, but some of those "loopholes" will likely be removed in the 2014 NEC.

 

[topgone]

Helo, I'm a newbie studying about power system protection. My question is:

http://www.hv-eng.com/OvercurrentCoord.pdf


In the above link, at page 52 at the "Coordination Pop Quiz #3" slide, the author states that:
"A main should never include an instantaneous setting"

I think he meant that for a main CB, you shouldn't include the instantaneous setting.
Can anyone explain why and what is the basis for this particular recommendation?
( Please give your reference / standard )

The usual practice in setting the protection of main circuit breakers has been to set the instantaneous trip at its maximum setting to better coordinate with downstream feeder breakers or defeat the instantaneous trip setting. Per UL and NEMA standards, instantaneous trip are required on MCCBs and ICCBs. LVPCBs are not required.

The 2011 NEC added an Art. 240.87 requiring that documentation are available to those authorized to design, install, operate or inspect the installation as to the location of the circuit breaker(s). Also:

Where a circuit breaker is utilized without an instantaneous trip, one of the following or approved equivalent means shall be provided:

• Zone-selective interlocking (ZSI)- breakers have communication logic to decide which breaker to trip on any fault location.
• Differential relaying - the current leaving and entering a device; the breaker or the bus are monitored and the problematic unit trips.
• Energy-reducing maintenance switching with local status indicator - built-in or a third-party logic device allowing switching from the normal protection mode to a maintenance mode; the maintenance mode settings with minimal delays in tripping times