Stopping short

In practice, the correct application of the various short-circuit ratings needs to be fully understood by the system designer to avoid leaving a circuit or equipment with inadequate protection. It is important to take full advantage of the capability of the devices and systems used to avoid over-engineering, and thus prevent the consequent unnecessary additional costs that this can cause.

To ensure the capability of equipment under short-circuit conditions, the system designer must be provided with the prospective fault level at the point of installation of each item to be used. This information is produced by a system protection study, for which proprietary software packages exist.

Fault levels can be defined in some or all of the following terms:

• prospective (available) short-circuit current;
  
• peak short-circuit current (Ip);
  
• steady-state short-circuit current (Ik).

Set ratings
A low voltage assembly such as a package substation, busbar trunking or distribution board, will have a short-circuit rating assigned by the manufacturer. This will be defined in terms of the maximum prospective fault level applicable at the point the assembly is connected into the system. The rating will have been determined by test and/or design calculations as specified in the assembly standard the applicable part of IEC 60439.

The standard provides the terminology that should be used to define the short-circuit rating of an assembly. This can be summarised as follows:

• rated short-time current of an assembly circuit (Icw);
  
• rated peak withstand current of an assembly circuit (Ipk);
  
• rated conditional short-circuit current of an assembly circuit (Icc).

An individual assembly may be assigned:

• a value of Icc only. If this is the case the assembly must be fed via the type and rating of the short-circuit protective device (scpd) specified by the manufacturer;
  
• values for both Icw and Ipk. Assemblies must be assigned values of Icw or Ipk together, since peak and rms fault currents are fundamental properties of a circuit;
  
• values for Icw, Ipk and Icc. In such cases Icc will generally be higher than Icw, therefore allowing the use of the assembly on a higher prospective fault level by use of the specified scpd;
  
• different values of Icc for individual circuit protective devices and/or system voltages;
  
• different values of Icw for different, short-circuit time periods, for example, 0·2, 1 or 3 s. Delay settings of upstream circuit-breakers must be co-ordinated within these withstand values.

Switchgear as a scpd
In terms of short-circuit capability, switchgear must be considered in respect of its function in each individual application.

A switching device has two functions on short-circuit: self-protection; and use as a scpd.

Using fused switchgear and fuses as scpd
Since the short-circuit breaking function in fused switchgear is provided by the fuses, it is the fuse characteristics that must be considered. These details are given in IEC 60269-1 and can be summarised as follows:

• breaking capacity of a fuselink;
  
• cut-off current;
  
• operating Joule integral (I2t).

When specifying a fuse to provide short-circuit protection to an assembly, the manufacturer will have verified by test and/or design calculations based on such tests, that the assembly is capable of handling the values of these parameters given by the fuse manufacturer.

Applying circuit-breakers as scpd
Moulded-case circuit-breakers (mccbs) and air circuit-breakers (acbs) are rated according to IEC 60947-2 in the following terms:

• rated short-circuit making capacity (Icm);
  
• two rated short-circuit breaking capacities:
  
• rated ultimate, short-circuit breaking capacity (Icu);
  
• rated service short-circuit breaking capacity (Ics);
  
• rated short-time withstand current (Icw).

A circuit-breaker can only be assigned a rated short-time withstand current if it is equipped with a time-delay over-current release.

All circuit-breakers that are designed to IEC 60947-2 will have assigned values of Icu and Ics. The prospective fault level in the circuit must not exceed Icu. The value of Ics indicates a fault level that the circuit-breaker can be subjected to with assured continuation of further service.

The following characteristics of circuit-breakers are not required by the standard IEC 60947 but they are applicable to short-circuit protection:

• cut-off current – the peak value reached by the current during the breaking operation in the instantaneous tripping range;
  
• operating Joule integral (I2t) - the integral of the square of the current over the operating time of the circuit-breaker on a short-circuit. This is sometimes referred to as energy let-through or I2t. It corresponds to the energy dissipated, thus represents the thermal effect on the circuit.

In simple studies, only the rms value of steady-state, short-circuit current (Ik) is quoted. In such cases, the peak current is assumed to be in a standard relationship to the rms current, determined by the overall power factor, in accordance with IEC standards.

The application of short-circuit protective devices (SCPDs) to circuit protection ie the protection of cables, is detailed in the installation rules, BS7671.

It is generally accepted that the selection of the protective device on the basis of the thermal protection of a cable automatically provides short-circuit protection up to the breaking capacity of the scpd in the case of non-time-delayed devices.

Short-circuit protection for lv assemblies
The prospective short-circuit current at the input to the switchboard is obtained from a system protection study.

This will be given as an rms value if the switchboard has:

• an Icw current value higher than the prospective current level. In this case the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the setting of releases upstream or at the incomer to the switchboard;
  
• an Icc rating higher than the prospective current level. Here the only requirement is to include the specified scpd in the circuit. This may be added in the circuit upstream or already be included as an incomer to the switchboard.

The prospective short-circuit current at the feed end of the busbar trunking is obtained from a system protection study. This will be given as an rms value if the busbar trunking has:

• an Icw current value that is higher than the prospective current level. In such cases the only requirement is to limit the time for which a short-circuit could persist to within the short-time value. This is achieved by the time-delay setting of overcurrent releases upstream;
  
• an Icw lower than the prospective current level, but an Icc rating that is higher than Ik. In these circumstances the only requirement is to include the specified scpd in the circuit upstream or in the end-feed unit.

The suitability of any given short circuit protective device, fuse or circuit-breaker may be derived from the cut-off current and Joule integral characteristics by comparison with proof-tested parameters supplied by the manufacturer. This often provides the most economical solution.

This article is an abbreviated version of a paper produced by the Engineered Systems Product Group (ESPG) of BEAMA Installation.

The paper is being put forward by the UK for use in International Electrotechnical Commission (IEC) matters and covers the fundamentals of short-circuit protection for engineered systems.