Technical Articles
Double Short Circuit Protection feature in Air Circuit Breakers provide the ability to set two levels of short circuit settings with adjustable time delays. This enables us to provide two values at which the circuit breaker is tripped at short circuit (under short time protection). The higher value of current usually corresponds to a much lower delay setting (hence lower tripping time), thus enabling a lower let through energy.
One of the prime considerations while designing electrical circuits and selecting protection devices is the let-through energy. The let-through energy is the energy passing through the Circuit Breaker when it is under short circuit. This energy causes a huge stress on the downstream devices and hence we, as electrical engineers, try to always minimize the let through energy as much as possible. These stresses reduce the life of the system components and also threatens the safety of the overall system.
Consider a circuit operating under normal condition with loads connected.
We have set the SC settings as follows: the pickup current for short time protection is 4 times the rated current. ( Is = 4 x In) and the delay time (ts) as 400msec.
Suppose a short circuit happens at a point A and let the magnitude of the Short Circuit Current be 8 times the rated current.
Now since the fault current (8 times the rated current) is greater than the pickup current (4 times the rated current), the breaker upstream of the fault will sense it and trip after its preset time delay (ie. 400msec).
As discussed earlier, the time for which the system remains under fault is very critical since the let through energy, the stress on the system, and to some extent even the overall system stability depends on it.
This time also needs to be sufficient for the selective co-ordination mechanism (ZSI) to work.
In the absence of Double Short Circuit Protection feature, the fault current that is 8 times the rated current will be seen by the CB immediately upstream to the fault with short time protection setting of 4 times the rated current. It will now wait through its delay of 400ms and then trip. Let the rated current be 1000A, then the let through energy flowing into the system during this fault is equal to 8kA x 8kA x 400 msec= 25.6 kA2 sec.
Circuit Breakers with Double Short Circuit Protection Feature can be set up with two group of short circuit parameters (pick up current and time delay).
In presence of a CB with this feature, the Pickup (Lo) can be set to 4 x In with a delay of 400msec and Pickup (Hi) can be set at 7 x In with a delay of 100msec.
So, when the fault current of 8 times the rated current will be seen by the CB immediately upstream to the fault, the Higher pickup (Hi) will be activated and the corresponding delay before tripping will only be 100msec.
This way, for In=1000A, the let through energy will be equal to 8kA x 8kA x 100msec =6.4 kA2 sec, which is significantly lower as compared to CBs without this feature (25.6 kA2 sec).
| Feature | Conventional Short Circuit | Double Selective–Short Circuit Protection |
| Pickup | 4 x In @ 400msec | S/C 1 : 4 x In @ 400msec |
| Pickup | S/C 2 : 7 x In @ 100msec | |
| Fault Magnitude | 8 x In | 8 x In |
| Let-through energy (for In = 1000A) | (8x1000)² x400 = 25.6 kA² sec | (8x1000)² x100 = 6.4 kA² sec |
Applications include a wide variety of switchgear industry wherein short circuit testing is done.
Starting from the MTX3.5, all Releases are equipped with Dual Short Circuit Protection wherein separate Short Circuit settings and time delays can be configured.
| Feature | MTX3.5 | MTX3.5EC | MTX3.5H | MTX4.5 |
| Protection: Enable/Disable | ✔ | ✔ | ✔ | ✔ |
| Double S/C ON/OFF | ✔ | ✔ | ✔ | ✔ |
| I2T ON/OFF | ✔ | ✔ | ✔ | ✔ |
| Pick-up Lo, Is = In x…. | 0.6 to 12 In in steps of 0.05 | |||
| Pick-up Hi, Is = In x…. | 0.6 to 12 In in steps of 0.05 | |||
| Delay Hi (ts) | 20-100-200-300-400 ms | |||
| Delay Lo (ts) | 20-100-200-300-400 ms | |||
| Pre-alarm | 0.5 to 0.95 in steps of 0.05 x Is | |||
| Cold-Pickup ON/OFF | ✔ | ✔ | ✔ | ✔ |
| Cold Delay | 100ms to 10s in steps of 100ms | |||
| Feature | OMEGA | MTZ | NW | MVS | Emax2 | Emax | Formula | 3WL | 3WT |
|---|---|---|---|---|---|---|---|---|---|
| Double Short Circuit | MTX3.5 onwards | Yes (Optional) | No | No | Ekip Hi-Touch | PR123 | No | ETU 76B | No |
Sandeep Pawar,
General Manager - Product ManagementElectrical Engineer with over 20 years of experience in Product Management and Application Engineering for Protection Relays, Power Quality Solutions, and Intelligent Motor Control Centres (MCCs). Possesses strong expertise in LV and MV power system protection schemes, motor protection, monitoring, and control applications. Well-versed in both conventional and intelligent motor management systems, with extensive knowledge of protection, control, and condition monitoring solutions.
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