Surge Protective Devices
Surge protective devices are items of equipment whose basic components
are voltage-controlled resistors (varistors, suppressors diodes) and/or
spark gaps (discharge paths). The function of surge protective devices is
to protect other electrical equipment and installations against impermissibly
high surges and/or to establish the equipotential bonding.
Surge protective devices are classified
a) upon their application in
- Surge protective devices for power supply systems and equipment
- Surge protective devices for IT systems and equipment
- Isolating spark gaps for earth-termination systems or for equipotential
bonding
b) upon their impulse current discharge capacity and their protective
effect in
- Lightning Current Arresters
- Surge Arresters
- Combined Lightning Current and Surge Arresters
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Nominal Voltage Un
corresponds to the nominal voltage of the system to be protected. The nominal
voltage is indicated in case of surge protective devides for IT installations
for type designation purposes. For ac voltages it is indicated as rms value.
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Max. Continuos Voltage Uc
(max. continous operating voltage) is the root mean square (rms) value of max.
voltage which may be applied to the correspondingly marked terminals of the
surge protective device during operation. It is the maximum voltage on the
SPD in the defined non-conductive state which ensures that this state is
regained after response and discharge.
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Nominal Load Current (Nominal Current) IL
is the highest permissible operating current which may be permanently
conducted via the correspondingly marked terminals.
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Max. Transmission Capacity
defines the max. HF capacity that can be transmitted via a coax surge protective
device without interfering with the protective component.
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Nominal Discharge Current In
is the peak value of an impulse current, waveform 8/20 μs, which the surge
protective device is rated for, according to a certain test programme.
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Max. Discharge Current Imax
is the max. peak value of the impulse current 8/20 μs, which can be
safely discharged by the device.
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Lightning Impulse Currentimp
is a standardised impulse current curve, with a waveform 10/350 μs. Its
parameters (peak value, charge, specific power) simulate the loads of natural
lightning currents.
Lightning current and combined lightning current and surge arresters must be
capable of discharging such lightning impulse currents several times without
consequential damage to the equipment.
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Voltage Protection Level Up
The voltage protection level of a surge protective device is the max.
instantaneous value of the voltage on the terminals of a surge protective
device, defined out of the standardised individual tests:
- Lightning impulse sparkover voltage 1.2/50 μs (100%)
- Response voltage at a steepness of 1kV/μs
- Residual voltage at a nominal discharge current Ures
The voltage protection level characterises the capability of a surge
protective device to limit surges to a residual level. If used in power networks,
the voltage protection level defines the location of use via the overvoltage
category according to DIN VDE 0110-1:1997-04 (EN 60664-1, IEC 60664-1). For surge
protective devices designed for protection of IT networks, the voltage protection
level has to be adjusted to the immunity of the equipment to be protected (DIN EN 61000-4-5: 2001-12).
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Disconnection Capacity / Follow Current Extinguishing Capability Ifi
The disconnection capacity is the uninterfered (prospective) rms-value of the
mains follow current, which can automatically be extinguished by the storage
protective device at the presence of Uc. It is proved at the
operating duty test according to E DIN VDE 0675-6/A1: 1996-03
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Short-circuit Withstand Capability
is the value of the prospective power-frequency short-circuit current controlled
by the surge protective device in case it is furnished with an upstream
backup fuse.
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Mains-side Overcurrent Protection / Backup Fuse
is an overcurrent protective device (e.g. fuse or circuit breaker), which is
installed outside of the surge arrester on the supply side to interrupt the
power-frequency follow currents, if the breaking capacity of the surge protective
device is exceeded.
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Operating Time ta
is the time passing until the automatic disconnection from the power supply at
a failure of the electrical circuit or equipment to be protected. The response
time is an application-specific value resulting from the intensity of the fault
current flowing and the characteristics of the protective device.
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Combined Impulse UOC
is generated by a hybrid generator (1.2/50 μs, 8/20 μs) with a virtual
impedance of 2 Ω. The open-circuit voltage of this generator is defined
as UOC. UOC is preferably indicated for SPDs Type 3.
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N-PE Surge Arresters
are surge protective devices exclusively designed for installation between
the N- and PE conductor.
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Operating Temperature Range
indicates the range where the devices can be used. In case of devices without
self-heating, it is equal to the ambient temperature range. The temperature
rise at devices with self-heating must not exceed the max. value indicated.
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Response Time tA
Response times generally characterise the response performance of the individual
protection elements used in surge protective devices.
Depending on the steepness du/dt of the impulse voltage or di/dt of the impulse
current, the response times can change within certain limits.
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Thermal Disconnection Device
Surge protective devices for power supply systems, which are furnished with
voltage-controlled resistors (varistors), mostly have an integrated disconnection
device isolating the surge protective device from mains at overloads and indicating
this state of operation.
The disconnection device reacts on the "joule heat" generated by an overloaded
varistor and disconnects the surge protective device from mains, if a certain
temperature is exceeded.
The disconnection device is designed to disconnect the overloaded surge
protective device in time to avoid a fire hazard. It is not designed to
ensure the protective measure of "protection at indirect contact".
The function of these terminal disconnection devices is tested via
simulated overloads/ageing of the SPD.
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Degree of Protection
The degree of protection IP corresponds to the subdivision into the degrees of
protection in accordance with DIN EN 60529 (VDE 0470 Part 1).
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Protective Circuit
Protective circuits ae multi-stage, cascaded protective devices. The individual
protection stages can consist of discharge paths, varistors, semi-conductor elements.
The energy coordination of the individual protection stages is realised with
decoupling elements.
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Series Impedance
is the impedance inthe direction of the signal flow between input and output
of the SPD.
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Bandwidth fG
defines the frequency-dependent preformance of an SPD. Bandwidths are frequencies
causing an insertion loss (aE) of 3 dB under certain test conditions
(see EN 61643-21:2000).
If nothing else is indicated, the value of the frequency refers to a 50 Ω system.
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Frequency Range
characterises the transmission band or the let-through frequency of an SPD
according to the described attenuation characteristics.
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Return Loss aR
indicates at high-frequency applications, how many rates of the "forward" wave
are reflected at the protective device ("transition point").
It is direct measure for rating the adjustment of the protective device
to the surge impedance of the system.
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Insertion Loss
At an indicated frequency, the insertion loss of a surge protective device
is defined via the relation of the voltage value at the installation site
before and after the insertion of the SPD. If no other value is indicated, the
value refers to a 50 Ω system.
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Sheild Attenuation
Relation of the power feeded into a coaxial cable to the power radiated through
the cable from the outer conductor.
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Protective Conductor Current IPE
is the current flowing through the PE connection when the surge protective
device is connected with the max. permanent voltage UC, corresponding
to the installation instructions and without load-side consumers.
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Measurement, Control, Instrumentation and Communication Applications
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