Protection Against Overcurrents in Electrical Distribution

ComPact NSX Circuit Breaker Overcurrent Trip Units

ComPact NSX circuit breaker trip units are designed to handle overcurrents (overload currents and short-circuit currents) and in certain cases, ground fault currents.

Pickup adjustments are calculated relative to the downstream circuit to be protected.
Time delay adjustments are calculated in relation to the protection management (coordination).

NOTE: The protection plan is based on coordination of the protections - and in particular on selectivity. This can be achieved by time delays (time-related selectivity) while complying with ammeter- and power-related selectivity rules.

There are two types of trip unit:

Thermal-magnetic trip units for ComPact NSX100-250 circuit breakers
MicroLogic electronic trip units for ComPact NSX100-630 circuit breakers

Standard Settings for Overcurrent Protections

Standard IEC/EN 60947-2 states the trip characteristics at the circuit breaker limits.

The following table summarizes the recommendations of standard IEC/EN 60947-2 for the circuit breaker protection function:

Protection function	Setting recommendations
Long-time protection	Long-time protection of the inverse time type (with I ²t constant): No trip for a current below 105% of Ir Trip in less than two hours for a current equal to: 120% of Ir for an electronic trip unit 130% of Ir for a thermal-magnetic trip unit For a higher fault current, the trip time is inversely proportional to the fault current value.
Short-time protection	Short-time protection is definite time: No trip for a current below 80% of the short time setting Trip for a current equal to 120% of the short time setting The trip time is: Less than 0.2 s for a short time protection with no time delay Equal to the value of the time delay tsd for a protection with time delay
Instantaneous protection	Instantaneous protection is definite time: No trip for a current below 80% of the instantaneous setting Trip for a current equal to 120% of the instantaneous setting The trip time is less than 0.2 s.

Protection function

Setting recommendations

Long-time protection

Long-time protection of the inverse time type (with I ²t constant):

No trip for a current below 105% of Ir
Trip in less than two hours for a current equal to:
- 120% of Ir for an electronic trip unit
- 130% of Ir for a thermal-magnetic trip unit

For a higher fault current, the trip time is inversely proportional to the fault current value.

Short-time protection

Short-time protection is definite time:

No trip for a current below 80% of the short time setting
Trip for a current equal to 120% of the short time setting

The trip time is:

Less than 0.2 s for a short time protection with no time delay
Equal to the value of the time delay tsd for a protection with time delay

Instantaneous protection

Instantaneous protection is definite time:

No trip for a current below 80% of the instantaneous setting
Trip for a current equal to 120% of the instantaneous setting

The trip time is less than 0.2 s.

The Neutral Conductor

The installation rules closely define the type of protection to be used having regard to:

Potential overcurrents (overloads and short-circuits).
Conductors to be protected.
Simultaneous cutoff of all conductors including the neutral conductor (multi-pole breaking).

NOTE: All three of the phase conductors must be protected at all times. The neutral conductor (if it is distributed and identical to the phases in size, that is, full neutral) is normally protected by the phase protection.

Description of the Neutral Protection

The neutral must have specific protection if:

It is reduced in size compared to the phases.
Nonlinear loads generating third order harmonics and multiples thereof are installed.

It may be necessary to cut off the neutral for functional reasons (multiple source diagram) or safety reasons (working with power off).

To summarize, the neutral conductor can be:

Non-distributed (3P).
Distributed, not cut off and not protected (3P).
Distributed, not cut off but protected (3P with ENCT option). Refer to DOCA0141EN, ComPact NSXMicroLogic 5/6/7 Electronic Trip Units - User Guide.
Distributed, cut off and protected (4P).

ComPact NSX trip units are suitable for all protection types.

ComPact NSX	Possibilities	Neutral protection
3P	3P, 3D	None
3P + ENCT	3P, 3D	None
	3P, 3D + N/2	Half neutral
	3P, 3D + N	Full neutral
	3P, 3D + OSN ⁽¹⁾	Oversized neutral
4P	4P, 3D	None
	4P, 3D + N/2	Half neutral
	4P, 4D	Full neutral
	4P, 4D + OSN ⁽¹⁾	Oversized neutral
(1) OSN (OverSized Neutral) protection is used when high third harmonic (and multiples of the third harmonic) currents are present. OSN protection is installed on MicroLogic 5, 6, and 7 trip units. Refer to DOCA0141EN, ComPact NSXMicroLogic 5/6/7 Electronic Trip Units - User Guide. P: Breaking pole, D: Pole protected by the trip unit, N: Neutral protection.