Just because a product has been proven to cope with a short circuit fault, it may not mean quite what you think.
There is confusion in the market about the short circuit rating of devices, components and assemblies. Often this is not deliberate, but is a consequence of devices, assemblies and the system having different functions and in turn requiring different standards.
There are a number of terms and symbols that refer to different types of Short Circuit Current (Ic). This article explains the basic principles and the applications of these alternatives with the aim to establish clarity.
The Short Circuit.
This is where a circuit is made between one phase and another, or earth, without there being a load between them. This enables a high magnitude of current to flow between the phases or down to earth.
In our industry, short circuits are rarely deliberate. Normally they occur when an external cable is damaged, or during installation and maintenance when stray cables, armouring or tools end up making the circuit.
When testing to IEC Standards, the short circuit is required to be consistent and repeatable. Therefore a bolted connection is made. Depending on the short circuit current rating of the circuit, either solid bars or cables are used to connect the phases, or earth, together.
The Short Circuit Current.
The rating or size of the short circuit current is dependent on the overall electrical system, or to be more specific, the electrical supply to the assembly. The most popular methods to supply an assembly is via either a transformer or a generator or a combination of the two.
The ability of the supply to provide a short circuit is dependent on a number of factors. With a transformer it is the kVA, secondary voltage and impedance which will have the most significant impact. With a generator, it is the combination of the engine and alternator size.
These factors, along with the size of the conductors supplying the assembly, both Cross Sectional Area (CSA) and their length will directly impact the magnitude of the short circuit able to be supplied to the assemblies incoming terminals. This rating of short circuit is known as the Prospective Short Circuit Current (Icp).
It therefore seems perfectly sensible to say that the assembly connected to an electrical system should be able to cope with the prospective fault.
Clearing a Short Circuit Current.
The next reasonable stage is to say that if a short circuit occurs and the prospective short circuit current flows, then you would want to stop it from flowing. This is where a Short Circuit Protective Device (SCPD) comes into use. The most common SCPDs are fuses, Air Circuit Breakers (ACB), Moulded Case Circuit Breakers (MCCB) and Miniature Circuit Breakers (MCB). Each type of device will have its breaking capacity declared as part of its product details.
Because fuses have a single operational function it is declared as a single rating. Circuit Breakers however have an additional complication as they have two declared ratings. One is described as the Ultimate Breaking Capacity (Icu), the other is the Service Breaking Capacity (Ics) which is normally declared as a percentage of the Ultimate Breaking Capacity. For this document we are not wishing to focus on the devices; the key point is that the device’s ultimate breaking capacity should be equal or greater than the prospective short circuit current of the electrical system.
Co-ordination and Selection of Devices.
It is expected that the assembly manufacturer selects the appropriate SCPD to protect certain circuits within and supplied by the assembly. The selection of the devices and setting of the devices’ operational times will have a fundamental impact on the overall effectiveness of the assembly.
For example, if a short circuit occurred in a small outgoing circuit, you would want the nearest (upstream) device to clear the fault. You would not expect the entire assembly to be blacked out due to the main incoming device operating.
Depending on the use of the assembly, the main incoming device may need to be set to operate many 100ths of a second after the initial short circuit. It is important to ensure that the rest of the assembly, in particular the busbar system, can cope with this duration.
Short Circuit Ratings of Busbar Systems.
The busbar system is an integral part of the overall assembly. It is essential that it is designed to cope with the electro-mechanical and thermal effects of a short circuit fault and the mechanical loads of holding the immediate busbar run in addition to adjacent connections to devices and distribution busbars.
There are two short circuit ratings that can be considered within an assembly. One is the rating that is associated with a SCPD, this is where the short circuit current will initially flow, and then be cleared by the protective device. This rating is known as the Conditional Short Circuit Rating (Icc). This rating is not given a time duration and specific care should be taken to ensure that the Conditional Short Circuit Rating is less than the SCPD’s rating but not less than the Prospective Short Circuit Current, particularly if the device can be time delayed. Because of the complexities of ensuring appropriate selection is made, the second alternative rating, the Short Circuit Withstand (Icw) is often specified.
This rating should be made up of 2 parts: firstly the RMS rating in kA and secondly a duration. There is no international standard for the timing, however times of 0.5, 1 and 3 seconds are regularly used.
In addition to the above, there is a further key difference when testing the Short Circuit Withstand test: the SCPD, if in circuit, must not operate, which in turn ensures that the full energy of the short circuit has to be contained by the busbar system. This includes the Rated Peak Withstand Current (Ipk), which is a surge of current that occurs on one phase of the system in the first full cycle, as well as the changes of electro-magnetic forces that occur through the number of cycles seen during the test. The number of cycles is dependent on the frequency of the supply and duration of the test. For example, if the test supply is 50Hz and the duration is 3 seconds, there will have been 150 cycles.
Icp = Prospective Short Circuit Current – The fault level available at the incoming terminals of the assembly.
Icc= Short Circuit Conditional Current – The fault level that an assembly can cope with if a SCPD is in circuit and operates.
Icw= Short Circuit Withstand Current – The fault level and duration that an assembly can cope with without the aid of a SCPD.
Ipk= Peak Short Circuit Withstand – The initial surge that occurs in the first cycle of the AC Short Circuit.
The Icp should not be greater than the breaking capacity of the SCPD, and the assembly should be capable of withstanding the duration that the SCPD can be adjusted up to.
Watch videos of short circuit tests here.