Overload capability

The power units of the Line Modules, Motor Modules and Power Modules are designed for brief overloads, i.e. the Modules are capable of supplying more than the rated current I_rated for short periods. In this instance, the thermal capacity of the heat sink is utilized, allowing for the relevant thermal time constants. The power semiconductors and actual current sensing circuit are rated for a maximum current I_max which must not be exceeded. The overload capability is determined by I_max, I_rated and the thermal time constants. A number of characteristic duty cycles are defined in the technical specifications for the power units. The SIZER for Siemens Drives engineering tool calculates the load on the basis of a specified duty cycle with optional time characteristic and then identifies the power unit which is required.

The thermal time constant of a power semiconductor chip is typically within the range of 100 ms. With frequencies below 10 Hz, the overload capacity is therefore limited. The software takes account of these limitations by means of a thermal model and protects the devices against overload in all operating states. It must be noted, especially at frequencies around 0 Hz, that the specified rated current I_rated is the root-mean-square value of a sinusoidal current. If the frequency of the three-phase system is reduced to 0 Hz, a pure direct current flows in all phases at standstill. The root-mean-square value of this direct current can reach the peak value of the sinusoidal current depending on the phase relation.

The output current in this state is greater than the rated current I_rated by a factor of v2. The individual motor terminals and cables are designed thermally for the rated current in normal operation, so the devices are protected against this overload while taking account of the thermal time constant.

Derating characteristics

The power units can be loaded with rated current or power and the specified pulse frequency up to an ambient temperature of 40 °C (104 °F). The heat sink reaches the maximum permissible temperature at this operating point. If the ambient temperature increases above 40 °C (104 °F), the resulting heat loss must be reduced to prevent the heat sink from overheating.

At a given current, the heat loss increases in proportion to the pulse frequency. The rated output current I_rated must be reduced to ensure that the maximum heat loss or heat sink temperature for higher pulses frequencies is not exceeded. When the correction factor k_f for the pulse frequency is applied, the rated output current I_ratedf which is valid for the selected pulse frequency isadjusted.

When configuring a drive, please note that power units may not be capable of supplying the full current or power in the temperature range between 40 °C (104 °F) and 55 °C (131 °F). The power units measure the heat sink temperature and protect themselves against thermal overloading at temperatures > 40 °C (104 °F).

The air pressure, and therefore air density, drops at altitudes above sea level. At these altitudes, the same quantity of air does not have the same cooling effect and the air gap between two electrical conductors can only insulate a lower voltage. Typical air pressure values are:

0 m (0 ft) above sea level: 100 kPa

2000 m (6562 ft) above sea level: 80 kPa

3000 m (9843 ft) above sea level: 70 kPa

4000 m (13123 ft) above sea level: 62 kPa

5000 m (16404 ft) above sea level: 54 kPa

At installation altitudes above 2000 m (6562 ft), the line voltage must not exceed certain limits to ensure that surge voltages can be insulated in accordance with EN 60664?1 for surge voltage category III. If the line voltage is higher than this limit at installation altitudes > 2000 m (6562 ft), measures must be taken to reduce transient category III surge voltages to category II values, e.g. equipment must be supplied via an isolating transformer.

In order to calculate the permissible output current or power, the derating factors must be multiplied for the effects described above. The derating factor k_I for current as a function of installation altitude can be offset against the derating factor k_T for ambient temperature. If the result of multiplying derating factor k_T by derating factor k_I is > 1, then the calculation must be based on a rated current of I_rated or I_ratedf. If the result is < 1, then it must be multiplied by the rated current I_rated or I_ratedf to calculate the maximum permissible continuous current. The derating factor k = k_f ? k_T ? k_I calculated by this method to obtain the total derating value must be applied to all current values in the specified duty cycles I_rated, I_H, I_L).

The derating characteristic curves of Power Modules, Line Modules and Motor Modules can be found in the technical specifications of the relevant Modules (see chapter SINAMICS S120 drive system).

Examples of derating characteristic curves and calculation of the permissible output current:

Current derating as a function of the ambient temperature

Current derating as a function of the installation altitude

Voltage derating as a function of the installation altitude

Example 1

A drive system is to be operated at an altitude of 2500 m (8202 ft) at a maximum ambient temperature of 30 °C (86 °F) and rated pulse frequency.

Since the ambient temperature is below 40 °C (104 °F), a compensation calculation (installation altitude/ambient temperature) can be applied.

Installation altitude 2500 m (8202 ft): Derating factor k_I = 0.965, k_U = 0.94

Max. ambient temperature 30 °C (86 °F): Derating factor k_T = 1.133

k_I ? k_T = 0.965 ? 1.133 = 1.093 ? 1.0 due to installation altitude/ambient temperature compensation

k = k_f ? (k_I ? k_T) = 1.0 ? (1.0) = 1.0

Result: Current derating is not required.

However, IEC 60664?1 stipulates that voltage derating is required.

The units in voltage range 380 V to 480 V can be operated up to a voltage of 480 V x 0.94 = 451 V, and the units in voltage range 660 V to 690 V up to 690 V ? 0.94 = 648 V.

Example 2

When a drive line-up is configured, a Motor Module with the order number 6SL3320?1TE32?1AA0 is selected (rated output current 210 A, base load current for high overload 178 A). The drive line-up is to be operated at an altitude of 3000 m (9843 ft) where ambient temperatures could reach 35 °C (95 °F) as a result of the installation conditions. The pulse frequency must be set to 4 kHz to provide the required dynamic response.

Installation altitude 3000 m (9843 ft): Derating factor k_I = 0.925, k_U = 0.88

Max. ambient temperature 35 °C (95 °F): Derating factor k_T = 1.066

k_I ? k_T = 0.925 ? 1.066 = 0.987 ? insufficient installation altitude/ambient temperature compensation

k = k_f ? (k_I ? k_T) = 0.82 ? (0.925 ? 1.066) = 0.809

Result: Current derating is required.

Where these boundary conditions apply,

• the max. permissible continuous current of the Motor Module is: 210 A ? 0.809 = 170 A
• the base-load current for high overloading is: 178 A ? 0.809 = 144 A

IEC 60664?1 stipulates that voltage derating is required.

The selected unit can be operated up to a voltage of 480 V 3 AC ? 0.88 or 720 V DC ? 0.88 = 422 V 3 AC or 634 V DC, i.e. under these conditions a 400 V asynchronous motor an be operated without restriction. Due to the installation altitude, however, derating might be required for the asynchronous motor (induction motor).

Selection of the Power Module or Motor Module

The Motor Module is selected initially on the basis of standstill current I_0 100 K (rated current for winding temperature rise 100 K) for synchronous motors and the rated current I_rated for asynchronous motors (induction motors), and is specified in the motor description. Dynamic overloads, e.g. during acceleration, must be taken into account by duty cycles and may demand a more powerful Power Module or Motor Module. In this context, it is also important to remember that the output current of the Power Module or Motor Module decreases as a function of installation altitude, ambient temperature and pulse frequency setting (see explanations of derating characteristics).

For an optimum configuration, the effective motor current I_load calculated from the duty cycle is replicated on the Power Module or Motor Module. The following must apply:

I_{rated, Module} ? I_load

I_{rated, Module} = permissible continuous current of Power Module or Motor Module taking derating characteristics into account

The Power Modules or Motor Modules can be required to supply a higher output current for specific time periods. The characteristics or overload capability must be noted (see chapter SINAMICS S120 drive system) when modules are engineered for overload.

The SIZER for Siemens Drives engineering tool is capable of performing precise overload calculations.

Rated current – permissible and non-permissible motor/converter combinations

• Motor rated current higher than rated output current of the Power Module or Motor Module:
  In cases where a motor with a higher rated current than the rated output current of the Power Module or Motor Module is to be connected, the motor will only be able to operate under partial load. The following limit applies:
  The short-time current (=1.5 ? base-load current I_H) should be higher or equal to the rated current of the connected motor.
  If this dimensioning rule is not adhered to, the low leakage inductance of large motors causes current peaks which may result in a drive system shutdown or in a continuous output limiting by the internal protective electronic circuitry.
• Motor rated current significantly lower than rated output current of the Power Module or Motor Module: With the vector control system used, the rated motor current must equal at least 1/8 of the rated output current of the Power Module or Motor Module. With smaller motor currents, the drive can be operated in V/f control mode.

Using pulse width modulation, the Power Modules or Motor Modules generate an AC voltage to feed the connected motor from the DC voltage of the DC link. The magnitude of the DC link voltage is determined by the line voltage and, in the case of a Motor Module, by the Line Module used and thus the maximum possible output voltage (see chapter SINAMICS S120 drivesystem). The speed and loading of the connected motor define the required motor voltage. The maximum possible output voltage must be greater than or equal to the required motor voltage; it may be necessary to select a motor with a different winding.

It is not possible to utilize all modes of pulse width modulation when a sine-wave filter is connected. The maximum possible output voltage (see sine-wave filter) is lower as a result.

Long motor cables

Using pulse width modulation, the Power Modules or Motor Modules generate an AC voltage to feed the connected motor from the DC voltage of the DC link. Capacitive leakage currents are generated in clocked operation and these limit the permissible length of the motor cable. The maximum permissible motor cable length is specified for each Power Module or Motor Module in the component description.

Motor reactors limit the rate of rise and magnitude of the capacitive leakage currents, thereby allowing longer motor cables to be used. The motor reactor and motor cable capacitance form an oscillating circuit which must not be stimulated by the pulse patternof the output voltage. The resonant frequency of this oscillating circuit must therefore be significantly higher than the pulse frequency. The longer the motor cable, the higher the cable capacitance and the lower the resonant frequency. To provide a sufficient safety margin between this resonant frequency and the pulse frequency, the maximum possible motor cable length is limited, even when several motor reactors are connected in series. The maximum cable lengths in combination with motor reactors are specified in the technical specifications for the motor reactors.

Where a long motor cable is required, a higher rating of power unit must be selected or the permissible continuous output current I_continuous must be reduced in relation to the rated output current I_rated.

1) Up to 70 m (230 ft) always permissible.

The permissible cable length for an unshielded motor cable is 150 % of the length for a shielded motor cable.

Motor reactors can also be installed in order to permit the use of longer motor cables.

Line Modules

In multi-axis drive applications, a number of Motor Modules are operated on a common DC link, which is supplied with power by a Line Module.

The first task is to decide whether a Basic Line Module, Smart Line Module or an Active Line Module will be used. On the one hand, this depends on whether the drive must be capable of regenerative feedback to the supply and, on the other hand, whether the power supply infeed is to be unregulated and therefore dependent on the power supply voltage, or regulated to a constant DC link voltage.

The chassis format units are available in the 380 V to 480 V voltage range, but also include units in the 500 V to 690 V range. Basic Line Modules are designed for infeed operation only. Active Line Modules have regulated infeeds which feature a step-up function.

In order to calculate the required DC link power and select the correct Line Module, it is important to analyse the entire operating sequence of the drive line-up connected to the DC link. Factors such as partial load, redundancies, duty cycles, coincidence factors and the operating mode (motor / generator mode) must be taken into account.

The DC link power P_d of a single Motor Module is calculated from the shaft output P_mech of the motor and the efficiency of the motor ?_m and Motor Module ?_wr.

The following applies in motor mode: P_d = P_mech / (?_m ? ?_wr)

The following applies in generator mode: P_d = P_mech ? ?_m ? ?_wr

The motor and generator outputs must be added with the corresponding sign in order to calculate the total DC link power. For the power calculation, the DC link voltage V_d can be assumed to be constant. The required DC link current is therefore calculated as I_d = P_d/V_d

Basic Line Modules

The DC link voltage V_d of the Basic Line Modules is load-dependent. Under no-load conditions, the DC link is charged to the line voltage crest value V_L, i.e. V_d = v2 ? V_L, e.g. V_d = 566 V when a 400 V supply system is connected.

Under load conditions, the DC link voltage reaches the average value of the rectified line voltage applied to the terminals. This average value is determined by the line voltage x factor 1.35. Owing to the voltage drop across the line reactor and in the line feeder cable, the DC link voltage under full load conditions is slightly lower than the theoretical value. In practice, the range of the DC link voltage V_d is as follows:

1.41 ? V_L > V_d > 1.32 ? V_L (no load > rated output)

Smart Line Modules

The DC link voltage V_d of Smart Line Modules is regulated to the average value of the rectified line voltage V_L, i.e. V_d ? 1.35 ? V_L

Due to the voltage drop across the line reactor and in the line feeder cable, the DC link voltage decreases in motor operation and increases in generator operation. The DC link voltage V_d thus varies within the same range as on drives with a Basic Line Module:

1.41 ? V_L > V_d > 1.32 ? V_L (rated output generator > rated output motor)

Active Line Modules

The DC link voltage V_d is regulated to an adjustable value (Active Mode). An Active Line Module can also be switched to Smart Mode and then operates like a Smart Line Module. In Active Mode, the Active Line Module draws a virtually sinusoidal current from the supply system.

The rated infeed power of the Line Module refers to a line voltage of 380 V, 400 V or 690 V (690 V applies only to chassis format Line Modules). The output power of the Line Modules may be affected if they are operated on line voltages other than those stated above.

Depending on the ambient conditions (installation altitude, ambient temperature), the rated infeed power of the Line Modules may need to be reduced (see chapter SINAMICS S120 drive system).

The coincidence factor takes into account the time characteristic of the torque for each individual axis.

On the basis of these principles, the following procedure can be used to dimension the Line Module:

The following factors must also be taken into account when dimensioning the DC link:

Braking operation

As device losses are important in motor mode, the dimensioning for motor mode is also applicable to generator mode. With respect to motor braking operation, check that the energy fed back into the DC link does not exceed the permissible peak load capability of the Line Module.

In the case of higher regenerative outputs and to control the "line failure" operating scenario, a Braking Module must be provided, the Smart or Active Line Module must be overdimensioned or the regenerative output reduced by longer braking times.

For the configuration of the "EMERGENCY STOP" operating scenario, the Line Module must either be overdimensioned or an additional Braking Module must be used, so that the DC link energy can be dissipated as quickly as possible.

Checking the DC link capacitance

During power-up, the Line Modules limit the charging current for the DC link capacitors. Due to the limits imposed by the precharging circuit, it is essential to observe the maximum permissible DC link capacitance values for the drive line-up specified in the technical specifications.

DC link pre-charging frequency

The pre-charging frequency of the DC link via a booksize format Line Module is calculated using the following formula:

For chassis format Line Modules, the maximum permissible DC link pre-charging interval is 3 minutes.

Special considerations for operation on Basic or Smart Line Module

Basic Line Modules and Smart Line Modules provide a lower DC link voltage than Active Line Modules. As a result, the following boundary conditions apply:

• When operating asynchronous motors (induction motors), a lower maximum motor power is available at high speeds at the same line voltage.
• On synchronous motors, a reduction in the dynamic drive characteristics must be expected at high speeds.
• On synchronous motors, the rated motor speed cannot be fully utilized when an overload capability is required.

Parallel connection of power units

Up to 4 Motor Modules or Line Modules in chassis format can be connected in parallel. Parallel connections can operate only in Vector Control mode.

Parallel connections may only include Motor Modules or Line Modules of the same type and with the same voltage and output ratings. Mixtures of different modules, e.g. Basic Line Modules and Active Line Modules, cannot be connected in parallel. The CU320?2, SIMOTION D4x5?2 or CX32?2 Control Unit can control only one drive object of type "Parallel connection Line Modules" and one of type "Parallel connection Motor Modules". It is assumed that all Line Modules or Motor Modules linked to the Control Unit are connected in parallel. A Control Unit can control,for example, the following components:

• 1 Line Module + 2 parallel-connected Motor Modules
• 2 parallel-connected Line Modules + 3 parallel-connected Motor Modules

Combinations such as the following are not permissible: 2 Line Modules + 2 Motor Modules connected in parallel + 1 Motor Module

In order to ensure symmetrical current distribution among all parallel-connected modules, inductances must be provided for subsystem decoupling. However, the current compensatory control cannot completely prevent asymmetrical current distribution, which means that the following derating factors apply to parallel connections:

Chassis format Line Modules

Line reactors are needed to decouple individual Basic Line Modules, while the appropriate Active Interface Modules are required to decouple Active Line Modules.

Parallel connection of Basic Line Modules using line reactors

Parallel connection of Active Line Modules using Active Interface Modules

Chassis format Motor Modules

Three-wire or four-wire cables should be used where possible to connect Motor Modules in parallel.

In this case, a minimum clearance of 50 mm (2 in) must be left between the cables of the individual subsystems. A three-phase system must be connected to each of the cables of equal length (U2, V2, W2). In order to ensure adequate decoupling between subsystems, the motor cables must be of a minimum length so as to provide the necessary inductance.

If the drive configuration cannot accommodate the minimum required cable length, the appropriate motor reactor for the Motor Module must be installed. Alternatively, motors with two separate winding systems can be used.

The latter option is preferable for drives with higher outputs, as the motor terminal boxes are subject to current limits in this case.

Parallel connection with identical motor cables of the required minimum length

Use of motor reactors

Asynchronous motor (induction motor) with two separate winding systems

Line harmonic distortion

The voltage drops across the impedance between the supply system and a load as soon as the load draws current. In a symmetrical three-phase supply system, this is the network impedance Z_n which is calculated from the impedance Z_s of the supply system and the line-side impedance Z_e of the load.

Effective impedances when a load is connected to a three-phase supply system

Z_n = Z_s + Z_e = R_s + j X_s + R_e + j X_e = R_n + j X_n

On a variable-speed drive, the line-side impedance Z_e is normally the total impedance provided by the line reactor and the feeder cable up to the PCC (Point of Common Coupling) for further loads. The ohmic component R_n is generally negligible as compared to the inductive component X_n. The inductance of an RI suppression filter is irrelevant for the purpose of this calculation, as this inductance is effective only for asymmetrical interference voltages, but not for a symmetrical line current.

If a load causes voltage drops across the impedance Z_s, this system perturbation has an impact at the PCC and thus also in the supply voltage to all other loads.

The voltage drop is proportional to current I_e and the impedance. To facilitate comparison of voltage drops under different supply and load conditions, the voltage drop is specified – normally at rated current – with reference to the phase voltage V_o. The calculation formula, e.g. for the per unit voltage drop u_k across an impedance Z is as follows:

u_k = Z ? I_e / V_o

Example 1:

A Power Module with rated line current I_e is directly connected to a low-voltage transformer and the PCC is the transformer connection terminal. The equation for the ratio between rated line current I_e of the Power Module and rated current I_rated of the transformer is I_e = 0.25 ? I_rated. The per unit voltage drop u_k of the 400 V transformer is 4 %. If the transformer is loaded with its rated current I_rated, the voltage drop across impedance Z_s is 9.2 V (corresponding to 4 % of the phase voltage V_o = 230 V).

u_k = (Z_s ? I_rated) / 230 V = 0.04

The following formula applies to the rated line current I_e of the Power Module: I_e = k ? I_rated

The per unit voltage drop across the transformer when loaded with I_e is thus: u_k = Z_s ? I_e / V_o = Z_s ? k ? I_rated / V_o

With the specified ratio between I_e and I_rated, the per unit voltage drop is calculated as u_k = 1 % or 2.3 V. In relation to the Power Module, this transformer therefore functions like a line impedance in accordance with u_k = 1 %.

The magnitude of system perturbation in converter systems is assessed on the basis of short-circuit power ratio R_sc:

R_sc = S_c? / P

According to this definition in accordance with EN 60146?1, P is the fundamental-wave apparent power drawn by the converter.
S_c? is the short-circuit power drawn from the mains in the event of a short-circuit on the terminals U1, V1, W1. Since the ohmic components of impedances are negligible in practice, the following applies Z_n ? j X_n

S_c? ? 3 ? V_o² / X_n

and thus R_sc ? 3 ? V_o² / (X_n ? P)

The short-circuit power ratio R_sc is therefore dependent on the current output power P of the converter and is determined by network impedance X_n.

If we assume the power to be P ? 3 ? V_o ? I_e = v3 ? V_rated ? I_e

the short-circuit power ratio R_sc is in inverse proportion to the per unit voltage drop u_k across the effective line impedance.

R_sc ? 3 ? V_o²/(X_n ? P) = 3 ? V_o²/(X_n ? 3 ? V_o ? I_e) = V_o/(X_n ? I_e) = 1/u_k

The short-circuit power ratio for example 1 is R_sc ? 100 if no line reactor is installed (Z_e = 0).

Note:

The term "short-circuit power ratio" as used in technical standards is not a harmonized definition. The short-circuit power ratio R_sce defined according to IEC 61000?3?12 is calculated from the short-circuit power S_SC at the PCC referred to the power S_equ = 3 ? V_o ? I_e consumed by the load.

Basic Line Modules and Power Modules are designed with a rectifier bridge on the line side. An inherent feature of the principle of rectification with load-side capacitance for DC link voltage smoothing are harmonics in the line current which result in a non-sinusoidal power input. The diagram shows the basic current waveform of a Power Module or Basic Line Module as a function of short-circuit power ratio R_sc.

Active Line Modules generate virtually no current harmonics (Active Mode) at all and are employed when system perturbation needs to be minimized, e.g. stipulation of IEEE 519 that THD (Total Harmonic Distortion) must be < 10 %.

The SIZER for Siemens Drives engineering tool calculates the line harmonic distortions on the basis of the supply data entered and lists it against the limit values of relevant standards.

Line current of a Basic Line Module or Power Module as a function of the short-circuit power ratio R_sc

The rms of the line current I_e for which the line-side components must be rated comprises fundamental wave I_e1 and the current harmonics, which increase in relation to the rise in short-circuit power ratio R_sc. If the DC link power P_d has been calculated (see Line Modules), the required line-side active power is a known quantity with Line Module efficiency, or the rectifier efficiency in the case of a Power Module. However, this active power is connected only with the current fundamental wave I_e1. The rms of the line current I_e is always greater than I_e1 as a result of the current harmonics. The following applies for a short-circuit power ratio R_sc = 100:

I_e ? 1.3 ? I_e1

The apparent power of a transformer selected to supply the drive must be greater than the drive power by a factor of about 1.3.

The harmonic currents produce only alternating power, but no active power. The following applies to the apparent power S on the line side:

S² = P² + Q₁² + D²

• with active component
  P = 3 ? V_o ? I₁ ? cos ?₁, produced solely by the current fundamental wave
• and apparent component
  Q₁ = 3 ? V_o ? I₁ ? sin ?₁ 
• and the distortion component

The ratio between active power and apparent power is referred to as total power factor ?:

Typical waveform of the line current with Power Modules and Line Modules

Line-side power options (main switch, fuses, line filters, etc.)

The following line-side power options are recommended for the drive configuration:

General overview of line infeed

The main switch may take various formats:

• Main and EMERGENCY STOP switch + fuse switch disconnector (with leading signal via auxiliary contact for trip mode)
• Load interruptor with fuses
• Circuit breaker

To protect the units against line-side surge voltages, it is advisable to install overvoltage protection directly at the infeed point (upcircuit of main switch). Overvoltage protection is essential in order to satisfy the requirements of Canadian standard CSA C22.2 No. 14?05. For examples of suitable surge voltage arresters, go to
http://www.raycap.com or http://www.dehn.de

Depending on the performance required, a fuse switch disconnector combined with a contactor or a circuit breaker can be provided as the main switch.

A line contactor can be used, for example, if the drive has to be disconnected from the line supply in the event of a fault or for remote tripping. Follow the instructions in the SINAMICS S120 Manuals to interlock the line contactor in the context of safety functions.

A line filter should be used on TN (grounded) systems to reduce line harmonic distortion.

SCCR (Short Circuit Current Rating)

In the USA, a rating plate must be attached to switchgear (referred to as Industrial Control Panels (ICP) in the USA), which indicates the short-circuit current rating (or overall panel SCCR) of the installation. Specification of the SCCR became essential when the National Electrical Code NEC 2005 came into force. The SCCR is calculated on the basis of UL508A Supplement SB4.

In order to ensure that the switchgear can withstand a short circuit in the main circuit, e.g. defects caused by current effects or fire, without sustaining serious consequential damage, the maximum possible short-circuit current may not exceed the SCCR value of the switchgear. The data of the transformer T2 which supplies the switchgear provide an adequate basis for making a rough calculation of the maximum possible short-circuit current at the installation site. Based on the rated current I_rated of the transformer and the relative short-circuit voltage u_k, the short-circuit current I_k is calculated according to the followingequation:rated of the transformer and the relative short-circuit voltage uk,the short-circuit current Ik is calculated according to the following equation:

I_k = I_rated/u_k

Example:

A transformer for 460 V 3 AC with a rated power of 1 MVA has a rated current of approximately 1255 A. The relative short-circuit voltage u_k of the transformer is 6 %. The maximum possible short-circuit current I_k directly at the output terminals of this transformer (low-voltage busbar) is 1255 A/0.06 ? 21 kA.

In order to calculate the short-circuit current, it is necessary to know the effective impedance of the supply cable and the transformers T1 and T2, as well as the short-circuit power of the line supply system. The maximum peak short-circuit current i_p is reached when the short circuit occurs at the voltage zero crossing. For method of calculating short-circuit currents, refer to IEC 60909?0.

The influence of the high-voltage and medium-voltage levels is slight and in most cases negligible for the selected example. When the effective impedance is taken into account, the maximum possible short-circuit current is lower than the previous value estimated from the data of the supply transformer, especially in the case of units which are not connected directly to busbars but over long cables to the transformer. Calculating the short-circuit current (peak short-circuit current) is complicated for systems which are supplied in parallel by multiple transformers and especially in the case of meshed systems.

The short-circuit current strength of the entire switchgear installation (overall panel SCCR) as specified on its rating plate is determined by the component in the main circuit with the lowest SCCR value.

Standard SCCR values for electrical equipment are specified in UL 508A Supplement SB4.2 (September 2005) and these can be used to calculate the overall panel SCCR. The following values are assumed for electric drives (motor controllers), for example:

The SCCR values of the Power Modules and Line Modules are higher than the standard SCCR values listed in table. The higher SCCR values apply only in combination with the fuses and circuit breakers specified in the manual! Fuses or circuit breakers can be exchanged for comparable types provided that the peak let-through current and breaking I²t value of the replacement type is not higher than those of the recommended type.