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Selectivity requirements

Selectivity between series protective devices is difficult to achieve unless the engineer responsible for specifying and purchasing the distribution equipment is familiar with available equipment features and functions. The engineer must also have a clear understanding of how sections of the distribution system should be removed from service during an overload or fault condition.

Proper selection and overcurrent coordination of protective devices
Proper selection and overcurrent coordination of protective devices (on photo: Molded case circuit breakers type Compact NSXm; credit: Schneider Electric)

Table 1 lists overcurrent relay curve types with associated applications, which are typically used in industry. Table 2 lists LV power circuit breaker trip functions with associated applications, which are again typically used in industry.

When evaluating the tripping characteristics for series protective devices on a Time-current curve (TCC), coordinating time intervals must be maintained based on the equipment under consideration.

Table 1 – Relay Curve Selection Chart

ApplicationFunctionsRelay Curve
Main Service51Extremely Inverse
Generator51VVery Inverse
Transformer50/51Very Inverse
Motor50/51Long Time
Capacitor50/51Short Time
Residual Neutral51Inverse
Neutral Ground51Inverse

Table 2 – LV Power Circuit Breaker Trip Function Chart

ApplicationLong TimeShort TimeInstantaneousGround Fault
Motor FeederYNYY
Transformer FeederYYYY
Generator FeederYYYY
Switchboard FeederYYNY
Panelboard FeederYYNY

The primary reason for coordinating time intervals is that MV relays and breakers are provided as separate, discrete components. Characteristic curves are provided by the relay vendor, and rated interrupting times are provided by the breaker manufacturer.

It is the responsibility of the engineer performing the coordination study to be aware of the overall relay-breaker TCC characteristics for the application under consideration.

There are two special cases concerning coordinating time intervals that warrant further discussion.

The first considers series fuses. The proper approach recommended in the standards and by fuse vendors is to maintain fuse ratios, not time margins on the TCC. For instance, consider the case of a 1600A Class L main fuse serving a 1000A Class L feeder fuse. When plotted on a TCC, the two curves will not touch.

The second case considers series LV power or molded-case circuit breakers. No coordinating time interval between series devices is required. Breaker characteristic curves incorporate breaker sensing and operating times. The purpose of the breaker total clear curve is to indicate that all poles in the circuit have been cleared.

Therefore, if the curves do not touch, selectivity is achieved.

LV Motor MCP Starter Feeder Unit

Industry standard phase overcurrent protection is provided in MCP starter units by two discrete components, an overload relay and an MCP. The MCP is a circuit breaker with the thermal element removed.

LV Motor MCP Starter Feeder Unit
LV Motor MCP Starter Feeder Unit

The overload and MCP characteristics are plotted on a phase TCC along with the motor starting curve and safe stall point, and the feeder damage curve.

The purpose of the overload-MCP combination is to allow the motor to start and run, and to protect the motor and cable from overloads and faults.

To accomplish this, the overload-MCP characteristics should be above and to the right of the motor starting curve, and to the left and below the motor safe stall point, the cable damage curve and amp rating.

Note it is not always possible to be below the cable amp rating due to overload tolerances. Suggested margins are listed below that have historically allowed for safe operation of the motor and cable while reducing instances of nuisance trips.

Title:Proper selection and overcurrent coordination of protective devices – Thomas P. Smith, P.E. at EPOWERENGINEERING
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Proper selection and overcurrent coordination of LV/MV protective devices
Proper selection and overcurrent coordination of LV/MV protective devices

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