Electrical Discrimination

[paulclayton1]

Can someone please explain what electrical discrimination is.

 

[rbulsara]

Normally the term discriminaiton is used for 'selectivity' or 'coordination' in operation of protective devices.

A properly coordinated electrical system will have proper discrimination so that when a fault occurs, the protective device closest to the fault will open, leaving rest of the system intact. For example, if there is a fault on a branch circuit, the brach circuit breaker in the distribution board will open, but not the main breaker feeding the distribution. Although the main breaker will also see the fault current but it will set so that it will wait untill a downstream device opens.

Certain relays have directional or distance features that will sense the direction or the distace of a fault location relative to the relay location and will only operate when the set criteria is made. This is also referred to as discrimination.

Hope this helps.

 

[RalphChristie]

From the Power System Protection series:

Discrimination, sometimes called selectivity, is the quality where a relay or protective system is enabled to pick out and cause to be disconnected only the faulty element.
Absolute discrimination are used on unit systems, they are able to detect and respond to an abnormal condition occuring only with the zone or the element they are specifically intend to protect.
Dependent (or relative) discrimination are used on non-unit systems. Their discrimination is not absolute, being dependent on the correlated or co-ordinated responses of a number of (generally) similar systems, all which respond to a given abnormal condition.

Discrimination is of two kinds, in one it refers to the ability of a device to discriminated as to the type of fault, so that it responds only to a specific type of fault condition; in the other it refers to the ability of the device to discriminate as to the location of the fault. Many discriminative systems of latter kind incorporate devices of the first-mentioned kind, and this applies both to unit and to non-unit systems

 

[sreid]

A discriminator is a FM (frequency modulation) dector if this is the contex in which "Discrimination" is used.

 

[electricpete]

Mechanical guys don't think electrical guys can figure out what a bearing is.

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[Skogsgurra]

Had to think about that, Pete. It isn't fair.

 

[jraef]

"Mechanical guys don't think electrical guys can figure out what a bearing is."

LOL Pete.
And what they don't understand is that although we know, we feel it is beneath us to care about something so mundane!

"Venditori de oleum-vipera non vigere excordis populi"

 

[PWR]

And then there is the Westinghouse (Now Eaton - Cutler Hammer) "Discriminator" which is available on certain low voltage power circuit breakers. It was originally an electromechanical device which allowed a breaker to trip instantaneously only if it was closed into a fault. After the introduction of solid state trip units, it became (and still is) "a circuit in the trip unit which determines at the time of a fault whether or not there has been any current flow in the primary circuit previous to the fault."

 

[mc5w]

There are also 600 volt and less circuit breakers such as from SquareD that can be set up with a restraint wire so that if a feeder circuit breaker is sensing and timing a ground fault it can tell an upstream circuit NOT to trip. This is primarily for either hospitals and other ultrareliable systems or for when all of the ground fault trips must be set for minimal time delay to avoid damage.