Coordination between circuit-breakers
Cascading (or Back-up protection)
The technique of “cascading” uses the properties of current-limiting circuit-breakers to permit the installation of all downstream switchgear, cables and other circuit components of significantly lower performance than would otherwise be necessary, thereby simplifying and reducing the cost of an installation
Definition of the cascading technique
By limiting the peak value of short-circuit current passing through it, a current-limiting CB permits the use, in all circuits downstream of its location, of switchgear and circuit components having much lower short-circuit breaking capacities, and thermal and electromechanical withstand capabilities than would otherwise be necessary. Reduced physical size and lower performance requirements lead to substantial economy and to the simplification of installation work. It may be noted that, while a current-limiting circuit-breaker has the effect on downstream circuits of (apparently) increasing the source impedance during short-circuit conditions, it has no such effect in any other condition; for example, during the starting of a large motor (where a low source impedance is highly desirable). The range of Compact NSX current-limiting circuit-breakers with powerful limiting performances is particularly interesting.
Conditions of implementation
In general, laboratory tests are necessary to ensure that the conditions of implementation required by national standards are met and compatible switchgear combinations must be provided by the manufacturer
Most national standards admit the cascading technique, on condition that the amount of energy “let through” by the limiting CB is less than the energy all downstream CBs and components are able to withstand without damage.
In practice this can only be verified for CBs by tests performed in a laboratory. Such tests are carried out by manufacturers who provide the information in the form of tables, so that users can confidently design a cascading scheme based on the combination of recommended circuit-breaker types. As an example, Figure H47 indicates the cascading possibilities of circuit-breaker types iC60, C120 and NG125 when installed downstream of current-limiting CBs Compact NSX 250 N, H or L for a 230/400 V or 240/415 V 3-phase installation.
|Type||Rating (A)||Icu (kA)||Reinforced breaking capacity (kA)|
- Note: for up-to-date cascading values, refer to manufacturer’s documentation, such as Schneider Electric’s Complementary Technical Information - Selectivity, Cascading and Coordination Guide
Advantages of cascading
The current limitation benefits all downstream circuits that are controlled by the current-limiting CB concerned.
The principle is not restrictive, i.e. current-limiting CBs can be installed at any point in an installation where the downstream circuits would otherwise be inadequately rated.
The result is:
- Simplified short-circuit current calculations
- Simplification, i.e. a wider choice of downstream switchgear and appliances
- The use of lighter-duty switchgear and appliances, with consequently lower cost
- Economy of space requirements, since light-duty equipment have generally a smaller volume
Principles of Selectivity
Selectivity is essential to ensure continuity of supply and fast fault localization.
Selectivity is achieved by overcurrent and earth fault protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected (see Figure H48).
Selectivity is required for installation supplying critical loads where one fault on one circuit shall not cause the interruption of the supply of other circuits. In IEC 60364 series it is mandatory for installation supplying safety services (IEC60364-5-56 2009 560.7.4). Selectivity may also be required by some local regulation or for some special application like :
- Medical location
- High-rise building
Selectivity is highly recommended where continuity of supply is critical due to the nature of the loads.
- Data center
- Infrastructure (tunnel, airport…)
- Critical process
From installation point of view: Selectivity is achieved when the maximum short-circuit current at a point of installation is below selectivity limit of the circuit breakers supplying this point of installation.
Selectivity shall be checked for all circuits supplied by one source and for all type of fault:
- Earth fault
When system can be supplied by different sources (Grid or generator set for instance) selectivity shall be checked in both cases.
Selectivity between two circuit breakers may be
- Total : up to the breaking capacity of the downstream circuit breaker
- Partial : up to a specified value according to circuit breakers characteristics Figure H49, H50 and H51
Different solution are provided to achieve selectivity based on:
Current based selectivity
see (a) of Figure H52
This method is realized by setting successive tripping thresholds at stepped levels, from downstream circuits (lower settings) towards the source (higher settings).
Selectivity is total or partial, depending on particular conditions, as noted above.
Time based selectivity
see (b) of Figure H52
This method is implemented by adjusting the time-delayed tripping units, such that downstream relays have the shortest operating times, with progressively longer delays towards the source. In the two-level arrangement shown, upstream circuit breaker A is delayed sufficiently to ensure total selectivity with B (for example: Masterpact with electronic trip unit).
Selectivity category B circuit breakers are designed for time based selectivity, the selectivity limit will be the upstream short time withstand value (Icw)
Selectivity based on a combination of the two previous methods
see (c) of Figure H52
A time-delay added to a current level scheme can improve the overall selectivity performance.
The upstream CB has two magnetic tripping thresholds:
- Im A: delayed magnetic trip or short-delay electronic trip
- Ii: instantaneous trip
Selectivity is total if Isc B < Ii (instantaneous).
Protection against high level short-circuit currents: Selectivity based on arc-energy levels
Where time versus current curves are superposed selectivity is possible with limiter circuit breaker when they are properly coordinated.
Principle: When a very high level short-circuit current is detected by the two circuit breakers A and B, their contacts open simultaneously. As a result, the current is highly limited.
- The very high arc-energy at level B induces the tripping of circuit breaker B
- Then, the arc-energy is limited at level A and is not sufficient to induce the tripping of A
This approach requires an accurate coordination of limitation levels and tripping energy levels. It’s implemented inside the Compact NSX range (current limiting circuit breaker), and between compact NSX and acti 9 range. This solution is the only one to achieve selectivity up to high short-circuit current with selectivity category A circuit breaker according to IEC60947-2.
Selectivity enhanced by cascading
Cascading between 2 devices is normally achieved by using the tripping of the upstream circuit breaker A to help the downstream circuit breaker B to break the current. By principle cascading is in contradiction with selectivity. But the energy selectivity technology implemented in Compact NSX circuit-breakers allows to improve the breaking capacity of downstream circuit-breakers and maintain high selectivity performance.
The principle is as follows:
- The downstream limiting circuit breaker B sees a very high short-circuit current. The tripping is very fast (<1 ms) and then, the current is limited
- The upstream circuit breaker A sees a limited short-circuit current compared to its breaking capability, but this current induces a repulsion of the contacts. As a result, the arcing voltage increases the current limitation. However, the arc energy is not high enough to induce the tripping of the circuit breaker. So, the circuit breaker A helps the circuit breaker B to trip, without tripping itself. The selectivity limit can
be higher than Icu B and the selectivity becomes total with a reduced cost of the devices
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.
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.
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.