INSTATENEOUS RELAY SETTINGS

[YIGO]

Hi everybody,

Instanteneous elements are sett off on 480V switchgears and MCC circuit breakers except for VFD CB in a power plant. Does anybody know what may be the reasons?
Thanks.

Yigo

 

[peterb]

Instantaneous trips have to coordinate with downstream protections. Unless there are transformers downstream which limit the current to secondary faults, instantaneous settings may not be possible - some time delay is required. A coordination study is the only real way to answer the question.

 

[DanDel]

Typically, Inst protection is not used in the Main device of a switchboard because it may trip along with a feeder device on a fault. I don't know about your specific applications.

 

[alehman]

Molded case circuit breakers all have an instanteous trip element in addition to inverse-time overcurrent elements. Some are adjustable, some are not. Molded case circuit breakers integral to motor starters frequently have only instantaneous trip. These are called motor circuit protectors.

The only type of circuit breaker that may not have instantaneous is a power breaker. In the U.S., that is only allowed in ANSI standard switchgear.

What type of circuit breaker supplies your VFD?

 

[YIGO]

Thanks All,

The CB are Cutler -Hammer MDS type. The trip units are Digitrip 520 type that include inst., short and long delays settings.
Peterb, there may be a coordination issue with inst. settings, in my understanding. the ins. settings can be the max. short circuit current at the point where the protection is located.
Dandel, if this happens, the coordination is not ensured.
Alehman, What settings inst. should be? what do you mean by power breaker?

Thanks again.

Yigo

 

[bigamp]

Peterb, DanDel and Alehman are all correct in what they say.

If you have downstream circuit breakers that have an instantaneous trip then the upstream protective device, say an air circuit breaker (ACB) should either have its instantaneous trip set to off (preferable) or it should be set to above the level of the prospective fault current so that it can not operate. This is to allow discrimination with the downstream device, pure and simple.

A typical situation will be an MCC where motor starters and feeders (to DB's say) all use moulded case circuit breakers (MCCB's). Depending on the trip unit fitted, MCCB's may have adjustible long time, short time and instantaneous trip settings. Generally with MCCB's the instantaneous trip can not be set to off. Some makes also have a so called "reflexive trip" where the contacts are forced apart by very high fault currents. The upstream device will be the incomer to the MCC and will typically be an ACB (in North American terminology, a power breaker?). This ACB will also have a trip unit with adjustible long time, short time and instantaneous trip settings. The instantaneous setting needs to be set as described, and you need to set the long time and short time settings to co-ordinate with the downstream MCCB's.

You can get the situation where the upstream breaker is an MCCB. Even though it should not be possible to set its instantaneous setting to off, you can still achieve discrimination with downstream devices provided the upstream MCCB is selected in accordance with discrimination charts provided by the manufacturer. Some manufacturers such as Merlin Gerin are really good at providing the necessary information to make a correct selection. I imagine CH is as well?

 

[alehman]

U.S. Power breaker = ACB. The MDS is a power breaker/ACB.

In the U.S., power breakers (only) may have an inst=off setting and only if installed in switchgear built per ANSI standards. No inst. on mains is commonly used to allow coordination with downstream devices which have inst. elements. Discrimination of inst. settings is not commonly considered in the U.S.

As bigamp suggested, the other breakers with inst=off are probably feeding other distribution boards or MCC's which have devices with inst.

It is possibe the VFD requires inst trip for it's own protection. Consult the drive manufacturer on this. If this is the only breaker protecting the motor, inst would be recommended to protect the motor as well.