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 H48 indicates the cascading possibilities of circuit-breaker types C60, DT40N, 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.
| kArms | ||||
| Short-circuit breaking capacity of the upstream (limiter) CBs | 150 | NSX250L | ||
| 70 | NSX250H | |||
| 50 | NSX250N | |||
 |
|
| ||
| Possible short-circuit breaking capacity of the downstream CBs (benefiting from the cascading technique) | 150 | NG125L | ||
| 70 | NG125L | |||
| 36 | NG125N | NG125N | ||
| 30 | C60N/H<=32A | C60N/H<=32A | C60N/H<=32A | |
| 30 | C60L<=25A | C60L<=25A(*) Quick PRD 40/20/8 |
C60L<=25A | |
| 25 | C60H>=40A C120N/H |
C60H>=40A C120N/H |
C60H>=40A C120N/H | |
| 20 | C60N>=40A | C60N>=40A | C60N>=40A |
(*) Quick PRD with integrated circuit-breaker as disconnector see chapter J
Fig. H48: Example of cascading possibilities on a 230/400 V or 240/415 V 3-phase installation
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 discriminative tripping (selectivity)
| Discrimination may be total or partial, and based on the principles of current levels, or time-delays, or a combination of both. A more recent development is based on the logic techniques. The Schneider Electric system takes advantages of both current-limitation and discrimination |
Discrimination is achieved by automatic 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 Fig. H49).
Fig. H49: Total and partial discrimination
Discrimination between circuit-breakers A and B is total if the maximum value of short-circuit-current on circuit B (Isc B) does not exceed the short-circuit trip setting of circuit-breaker A (Im A). For this condition, B only will trip (see Fig. H50).
Fig. H50: Total discrimination between CBs A and B
Discrimination is partial if the maximum possible short-circuit current on circuit B exceeds the short-circuit trip-current setting of circuit-breaker A. For this maximum condition, both A and B will trip (see Fig. H51).
Fig. H51: Partial discrimination between CBs A and B
Protection against overload : discrimination based on current levels
(see Fig. H52a)
This method is realized by setting successive tripping thresholds at stepped levels, from downstream relays (lower settings) towards the source (higher settings). Discrimination is total or partial, depending on particular conditions, as noted above.
Fig. H52a: Discrimination
As a rule of thumb, discrimination is achieved when:
- IrA/IrB > 2:
Protection against low level short-circuit currents : discrimination based on stepped time delays (see Fig. H52b)
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 discrimination with B (for example: Masterpact with electronic trip unit).
Fig. H52b: Discrimination
Discrimination based on a combination of the two previous methods (see Fig. H52c)
A time-delay added to a current level scheme can improve the overall discrimination performance.
The upstream CB has two high-speed magnetic tripping thresholds:
- Im A: delayed magnetic trip or short-delay electronic trip
- Ii: instantaneous strip
Discrimination is total if Isc B < Ii (instantaneous).
Fig. H52c: Discrimination
Protection against high level short-circuit currents: discrimination based on arc-energy levels
This technology implemented in the Compact NSX range (current limiting circuit- breaker) is extremely effective for achievement of total discrimination.
Principle: When a very high level short-circuit current is detected by the two circuits- breaker 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
As a rule of thumb, the discrimination between Compact NSX is total if the size ratio between A and B is greater than 2.5.
Current-level discrimination
This technique is directly linked to the staging of the Long Time (LT) tripping curves of two serial-connected circuit-breakers.
Fig. H53: Current discrimination
The discrimination limit ls is:
- Is = Isd2 if the thresholds lsd1 and lsd2 are too close or merge,
- Is = Isd1 if the thresholds lsd1 and lsd2 are sufficiently far apart.
As a rule, current discrimination is achieved when:
- Ir1 / Ir2 < 2,
- Isd1 / Isd2 > 2.
The discrimination limit is:
- Is = Isd1.
| Discrimination quality Discrimination is total if Is > Isc(D2), i.e. Isd1 > Isc(D2). This normally implies:
Current discrimination is normally used in final distribution. |
Time discrimination
| Discrimination based on time-delayed tripping uses CBs referred to as “selective” (in some countries). Implementation of these CBs is relatively simple and consists in delaying the instant of tripping of the several series-connected circuit-breakers in a stepped time sequence |
This is the extension of current discrimination and is obtained by staging over time of the tripping curves. This technique consists of giving a time delay of t to the Short Time (ST) tripping of D1.
Fig. H54: Time discrimination
The thresholds (Ir1, Isd1) of D1 and (Ir2, Isd2) comply with the staging rules of current discrimination.
The discrimination limit ls of the association is at least equal to li1, the instantaneous threshold of D1.
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Discrimination quality
A category circuit-breakers can be used with time-delayed tripping of the upstream circuit-breaker. This allows extension of current discrimination up to the instantaneous threshold li1 of the upstream circuit-breaker: Is = li1.
At this level, as continuity of supply takes priority, the installation characteristics allow use of B category circuit-breakers designed for time-delayed tripping. These circuit-breakers have a high thermal withstand (Icw ≥ 50% Icn for t = 1s): Is = Icw1. |
Note: Use of B category circuit-breakers means that the installation must withstand high electrodynamic and thermal stresses.
Consequently, these circuit-breakers have a high instantaneous threshold li that can be adjusted and disabled in order to protect the busbars if necessary.
Practical example of discrimination at several levels with Schneider Electric circuit-breakers (with electronic trip units)
"Masterpact NT is totally selective with any moulded-case Compact NSX circuit breaker, i.e., the downstream circuit-breaker will trip for any short-circuit value up to its breaking capacity. Further, all Compact NSX CBs are totally selective, as long as the ration between sizes is greater than 1.6 and the ratio between ratings is greater than 2.5. The same rules apply for the total selectivity with the miniature circuit-breakers Multi9 further downstream (see Fig. H55).
Fig. H55: 4 level discrimination with Schneider Electric circuit breakers : Masterpact NT Compact NSX and Multi 9
Energy discrimination with current limitation
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. The discrimination limit Is is consequently equal to the ultimate breaking current Icu B of circuit-breaker B acting alone, as cascading requires the tripping of both devices.
The energy discrimination technology implemented in Compact NSX circuit-breakers allows to improve the discrimination limit to a value higher than the ultimate breaking current Icu B of the downstream circuit-breaker. 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 discrimination limit can be higher than Icu B and the discrimination becomes total with a reduced cost of the devices
Natural total discriminitation with Compact NSX
The major advantage of the Compact NSX range is to provide a natural total discrimination between two series-connected devices if:
- The ratio of the two trip-unit current ratings is > 1.6
- The ratio of rated currents of the two circuit-breakers is > 2.5
Logic discrimination or “Zone Sequence Interlocking – ZSI”
| Discrimination 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 discrimination can be achieved with circuit-breakers equipped with specially designed electronic trip units (Compact, Masterpact): only the Short Time Protection (STP) and Ground Fault Protection (GFP) functions of the controlled devices are managed by Logic Discrimination. In particular, the Instantaneous Protection function - inherent protection function - is not concerned.
Settings of controlled circuit-breakers
- time delay: there are no rules, but staging (if any)of the time delays of time discrimination must be applied (ΔtD1 ≥ ΔtD2 ≥ ΔtD3),
- 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 discrimination even with circuit-breakers of similar ratings.
Principles
Activation of the Logic Discrimination function is via transmission of information on the pilot wire:
- ZSI input:
- low level (no downstream faults): the Protection function is on standby with a reduced time delay (y 0,1 s),
- 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 Fig. H56). 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 natural time delay (high level input). The circuitbreaker placed just above the fault does not receive any orders (low level input) and thus trips almost instantaneously.
Fig. H56: Logic discrimination.
| Discrimination quality This technique enables:
All the protection devices are thus virtually instantaneous,
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ru:Согласование характеристик автоматических выключателей zh:断路器间的配合
