Can you explain coordination between circuit-breakers, based on logic selectivity or “Zone Sequence Interlocking – ZSI” ?

Selectivity schemes based on logic techniques are possible, using CBs equipped with electronic tripping units designed for the purpose (Compact, Masterpact) and interconnected with pilot wires.

This type of selectivity can be achieved with circuit breakers equipped with specially designed electronic trip units (Compact, Masterpact): only the Short Time Protection (Isd, Tsd) and Ground Fault Protection (GFP) functions of the controlled devices are managed by Logic Selectivity. In particular, the Instantaneous Protection function is not concerned.

One benefit of this solution is to have a short tripping time wherever is located the fault with selectivity category B circuit breaker. Time based selectivity on multi level system implies long tripping time at the origin of the installation.

Settings of controlled circuit breakers

• time delay: Staging of the time delays is necessary at least for circuit breaker receiving a ZSI Input (ΔtD1 > trip time with no delay of D2 and ΔtD2 > trip time with no delay of D3)
• *
• thresholds: there are no threshold rules to be applied, but natural staging of the protection device ratings must be complied with (IcrD1 > IcrD2 > IcrD3).

Note: This technique ensures selectivity even with circuit breakers of similar ratings.

Principles

Activation of the Logic Selectivity function is via transmission of information on the pilot wire:

• ZSI input:
  • low level (no downstream faults): the Protection function is on standby with no time delay,
    *
  • high level (presence of downstream faults): the relevant Protection function moves to the time delay status set on the device.
• *
• ZSI output:
  • low level: the trip unit detects no faults and sends no orders,
    *
  • high level: the trip unit detects a fault and sends an order.

Operation

A pilot wire connects in cascading form the protection devices of an installation (see Figure H55). When a fault occurs, each circuit breaker upstream of the fault (detecting a fault) sends an order (high level output) and moves the upstream circuit breaker to its set time delay (high level input). The circuit breaker placed just above the fault does not receive any orders (low level input) and thus trips almost instantaneously.

Fig. H55 – Logic selectivity.

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Gepubliceerd voor: Schneider Electric Belgium