Short circuit

An electrical short is a practically unopposed connecting the two poles of an electrical voltage source ( or more generally, of two circuit points with usually different potential ), a voltage between these parts falls through the set to a value close to zero. The term refers to both the physical connection (no current flow ) and the event of the ( extreme ) current flow through this connection as soon as it or the voltage source to be activated.

In contrast to electrical connections, which serve to shield or control field, a high current may flow through a short-circuit connection, which often is a multiple of the operating current. This high short-circuit current comes through him hardly opposite ohmic resistance exhibitor (see Ohm's law ).

Intended shorts

The five safety rules to avoid electrical accidents suggested that before working on electrical systems, not only the outer conductor must be unlocked, but then optionally also a short circuit between the power is disconnected and outer conductors and ground potential must be established.

During maintenance on switchgear and overhead lines mobile and stationary earthing and short-circuiting devices are used. The mobile grounding devices in electrical installations include the grounding rod and grounding whip. These are created when switched off and after subsequent system testing for absence of voltage. This unintentional charging of the enabled and high-impedance system components are avoided. Among the stationary short -circuit protective devices include earthing switches that are installed in incoming feeders of medium voltage switchgear, often in combination with the switch-disconnector. Under certain conditions, the outgoing side earthing switch may be required. Instead of the earthing switch may, under certain conditions, so-called " fixed earthing points " be approved by the power company.

Large capacitors ( power capacitors ) must be handled briefly closed during storage and transportation, to ensure that they are unloaded and of them when touching the terminals poses no danger, even if a particular discharge is specified according to VDE. It is not enough to discharge a capacitor unique because can wander new charges from the dielectric to the electrode through the dielectric absorption.

ESD sensitive electronic components (MOSFET, IGBT, ICs) are often short-circuited before installation for transport and safe handling to avoid damage due to electrostatic discharge. Static electricity caused voltages of several thousand volts, which would otherwise destroy the components during unloading.

If large quantities of highly concentrated salt solution is injected into a tissue, this solution acts as a short circuit for the transmission of ( pain ) nerve impulses in this tissue; which as a method for local anesthesia - are used - especially at the dentist.

Accidental short-circuiting

If a short circuit between the poles of a battery or voltage-carrying phase conductors or an outer and the neutral conductor of a three-phase system the current reaches its maximum value, the initial short-circuit current. This current is limited only by the impedance of the line in series and the internal resistance Ri of the power source. The short-circuit current is therefore:

Where U is the voltage of the voltage source, the sum of the impedances Z ( active and reactive resistors) is in the short circuit current path. This very high initial short-circuit current is only a few milliseconds and then weakens to a sustained short-circuit current. It can be switched off by overcurrent protective devices.

Short circuits are usually caused by a defective or become insulation by a switching error in electrical systems or branch circuits. Shorts between all external conductors L1/L2/L3 called three-phase short circuits. All short-circuit currents are detected by guards and the current carrying conductors switched off by power switch or by fuses.

In underground cables, it may be troublesome to locate the short-circuit, since exposure of the core is often not possible over its entire length.

Causes and Types

A short circuit can be caused by:

• Insulation breakdown caused by, for example, by aging
• Insulation changes by constant stress on the insulation materials by high electric field possibly with partial discharges
• Due to overheating and subsequent softening or chemical changes in the insulation
• By influence of water ( it form leakage paths or the insulating material absorbs water )
• By mechanical damage to the insulation ( heavy-duty hand-held devices, construction sites )

A distinction is made between short- circuit caused by touching an energized body ( body contact ) and short-circuit with a fault resistance ( effective resistance and reactance ).

Short circuited via a fault resistance can cause arcing with a characteristic arc voltage. The arc behaves strongly non-linear, it does not limit the power and causes high temperatures ( 5,000 to 15,000 ° C) and interference voltages. Due to its thermal and ionizing effect other insulating components can be damaged.

Since the arc at each half-wave of the alternating current according to the zero crossing of the voltage extinguishes fires as at a higher voltage, it causes a phase shift of the current as in the case of a phase control.

In three-phase systems, a three-pole ( symmetric ) two-pole caused a short circuit ( between two of L1, L2 or L3) or a single-pole ( unbalanced, between L1 and ground facilities ).

The two -phase short -circuit is the one case in which flows in three-phase systems the largest short-circuit current, because the three-phase voltage source is loaded relatively low in spite of the short circuit and therefore in this short circuit still has the most energy reserves. There occur also to asymmetries, which can lead to surges in the other conductors.

The electrical equipment ( aluminum cables, circuit breakers, circuit breakers, current transformers, also carrying iron structures as well as the grounding wire) must therefore be dimensioned according to the maximum occurring two -phase short -circuit current. In this case (heat phenomena ) and dynamic ( magnetic force effects ) Short circuit current distinction is made between thermal phenomena.

Electrical regulations and guidelines for the calculation of short circuit current for electrical ( high voltage ) switchgear can be found in the VDE standard 0102.

Size of the short-circuit current

A high short-circuit current can only occur if the connection between is not active or reactive resistance more. If the resistance between the live conductors still a very low value, then one speaks of a " short-circuit -like" operation.

Example: between outer conductor ( eg L1 ) and neutral ( N) drive 230 V the leakage current to ground potential because of the opposite pole representing the neutral is bonded; between two phase conductors (such as L1 and L2) drive 400 V short-circuit current, as adjusted by the phase shift of 120 ° is higher by √ 3 rms (see three-phase alternating current).

This short circuit current during the short- circuit duration tk, among others by the internal resistance Ri of the voltage source ( prakt the secondary winding of the upstream local network transformer ), and round by the arc resistance at the short circuit point, the fault resistance at the short circuit point and the ladder resistors ( active and reactive resistance) of return conductor determines or limits.

The maximum expected short circuit current thus depends on the internal resistance and the line impedance of the power system and its rated voltage. Overload protection (circuit breaker, miniature circuit breakers, fuses ) must be able to turn off this current. It is located in house wiring nets at about 500 to 3000 A.

Consequences and countermeasures

Through a lack of limiting the short -circuit current and the short circuit similar error current may cause damage due to overheating ( more insulation damage, fires) over the wires or cables or electrical switchgear components, if they are not matched by the cross section of the wire nuts are protected.

In extreme cases, affected by the short -circuit wires can suddenly evaporate and cause an arc. If an arc on, it comes by radiant heat by metal splashes and the pressure wave to other hazards. Modern medium voltage switchgears are equipped with pressure relief valves that shut down the incoming feeder via a contact.

To prevent the consequences of short circuits are employed in low-voltage networks, a so-called circuit breaker and fuses various characteristics. Fuses must " burn out " with a high short-circuit current occurs while the short-circuit point as quickly as separate from the rest of the supply network. The disconnection must be done very quickly depending on the system (maximum in 1/10-Sekundenbereich ), depending on the characteristics of the scope (domestic installation, electronic protection, control protection, etc. ) differ in order to minimize the impact of the voltage break-in and the short circuit current low.

In the high and medium voltage networks, power system protection relays are used that can detect a fault and its place on the basis of current and voltage measurements and off by means of a circuit breaker, the corresponding parts of the network. For overhead lines, the so-called automatic reclosing (AR ) is used as a short-term " Astabfall " is not to cause a trip. It will be turned on when a triggering event or the circuit breaker after about 250 ms (short break) again; The error occurs as before, then the circuit breaker is switched on permanently in position from: AR successful - error is gone, AWE without success - error still exists. Also, traction power substations often lead a few seconds after a short-circuit one or more automatic reclosure is to allow for certain errors ( arc by lightning or birds) continued operation of the corresponding leg.

The mechanical strength, eg of exposed busbars must be measured in AC systems for the biggest shock occur short-circuit current. This short-term surge is to be linked through the shock factor κ with the initial short-circuit current. The factor κ is in the range 1-2 and is determined by the effective real and reactive resistance in the short-circuit path. The mechanical stresses show because of the power frequency dynamic behavior.

Particularly large forces occur at near- generator short circuit and cause mechanical damage to or - in the case of oil-filled transformers - are difficult to come erasable transformer fires. Power transformers are therefore often placed apart in the open and by corresponding concrete walls of the environment.

Short-circuit strength

Transformers to implement the level of the mains voltage (230 V) with protective low voltage below 50 V can be interpreted that the magnetic core has a sufficiently large leakage flux to limit the primary current in the secondary side short circuit. A similar principle is present in welding transformers, which are short-circuited on the secondary side for operational reasons.

Typical applications are bell transformers and small toys transformers, formally an inherently short -circuit proof to be construed in accordance with VDE 0551 and EN 60742. The primary current flow also needs to be limited to secure at least against short-circuit in the supply lines. The additional protection is usually through a thermal protection device that separates the primary current flow at a core temperature exceeded until cool.

Short circuit at the inputs and outputs of electronic devices

In order to test an electronic device to determine whether the input of a device applied disorders, it is often helpful to the inputs ( low frequency inputs, antenna socket) to "short circuit ". This is " engineer -speak ". Correct the expression, you would bridge the inputs, because there is no current will flow, however, induced noise signals to ground are derived. In this way you can narrow down the source of interference.

However, it is usually not possible without damage, an output ( eg speaker terminal of an amplifier, a transmitter antenna connector ) short circuit. Although such short circuits cause a hazard, but usually lead to overload and destroy the final or individual components ( transistors, etc. ).

Therefore amplifier outputs are often protected by various protection circuits shut down the amplifier in case of short circuit. Another possible security feature is possibly an output transformer is which is dimensioned such that in the event of a short circuit of the secondary winding, on which the load hangs, the maximum permissible output current is not exceeded.

Short circuit of batteries

Accumulators can depending on the type provide significant short-circuit currents, which can not only cause damage or destruction of the batteries but also to property and personal injury.

Automotive starter batteries ( lead-acid batteries ) can provide, for example, about 1000 A, thus it can come in the event of a short circuit in electric fire - a common cause of fire in motor vehicle accidents. Occurs when working on the battery terminals of the starter battery short circuit by a conductive tool or piece of jewelry ( metallic watchband or ring ) on, this burns and / or metal splashes can produce. Therefore, the rule is that the ground connection (-) always at one end and is last to join.

For redundant and diode- decoupled battery installations with dc voltage of 220 V are in two errors case an intentional short circuit: With a (-) ground faults to the battery 1 and a ( ) ground faults to the battery 2, or vice versa, creates an impermissibly high voltage ( by both earth faults caused serial connection of both batteries: max U1 U2 = 440 V).. Through a rigid connection between the two ( -) terminals of these two errors cause a short circuit to ground, thus turning an upstream overcurrent protection device from secure. Thus, an overload caused by improper voltage increase is avoided by the consumer.

Subtransienter initial short -circuit current

The subtransient initial short -circuit current is a term used in electrical engineering. This is a purely theoretical quantity in the short-circuit current calculation. It refers to the effective value of the AC component of the short-circuit current at the time of short-circuit occurrence. It is not used for mechanical assessment of current effects in the case of short circuit. For this consideration the peak short- circuit current is used.

Peak short- circuit current

The peak short-circuit current Is is decisive for the assessment and calculation of mechanical strength, in particular by:

• Generator windings
• Transformer windings
• Cable and pipe routes
• Electrical switchgear.

The peak short-circuit current is the maximum instantaneous value of the alternating current after fault inception and is expressed as peak value.

Peak short- circuit current   Factor 1.1 to 1.9, depending on the size of the source   Initial short circuit current

Protective devices such as circuit breakers or fuses are not designed for peak short- circuit current, but for the effective thermal short circuit current.

Standardize

• DIN EN 60909-0 VDE 0102:2002-07; Short-circuit currents in three- phase ac systems - Calculation of currents ( supplementary sheets 1, 3 and 4)
• DIN EN 60909-3 VDE 0102-3:2010-08; Short-circuit currents in three-phase networks - currents at Doppelerdkurzschluss and partial short -circuit currents to earth
• DIN EN 60865-1 VDE 0103:1994-11; Short-circuit currents; Calculation of effects - Definitions and calculation methods ( Supplement 1 )