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SPD terminology
Nominal Voltage
U
o
is the phase to neutral AC RMS
voltage of the mains system (derived from the
nominal system voltage) for which the SPD is designed.
U
o
is the voltage by which the power system is
designated - e.g. 230 V.
Maximum Continuous Operating Voltage
U
c
is the
maximum RMS voltage that may be continuously
applied to the SPD’s mode of protection e.g. phase to
neutral mode. This is equivalent to the SPD’s rated
peak voltage.
Temporary Overvoltage
U
T
is the stated test value of
momentary voltage increase or overvoltage that the
power SPD must withstand safely for a defined time.
Temporary overvoltages, typically lasting up to several
seconds, usually originate from switching operations or
wiring faults (for example, sudden load rejection,
single-phase faults) as well as mains abnormalities such
as ferro-resonance effects and harmonics.
Impulse Current
I
imp
is defined by three parameters, a
current peak with a charge and a specific energy
typically simulated with the 10/350 µs waveform to
represent partial lightning currents.
This waveform is used, with peak
I
imp
current value
stated, for the mains Type 1 SPD Class I test and
typically for data/telecom SPD Test Category D.
Nominal Discharge Current
I
n
is a defined nominal
peak current value through the SPD, with an 8/20 µs
current waveshape. This is used for classification of
mains SPDs (Class II test) and also for preconditioning
of SPDs in Class I and Class II tests. (Note: within
BS 7671,
I
n
is referred to as
I
nspd
).
Maximum Discharge Current
I
max
is the peak current
value through the SPD, with an 8/20 µs waveshape.
I
max
is declared for mains Type 2 SPDs in accordance to
the test sequence of the Class II operating duty test. In
general,
I
max
is greater than
I
n
.
Combined Impulse Test with Open Circuit Voltage
U
oc
is a hybrid 1.2/50 µs voltage test combined with an
8/20 µs current.
The test is performed using a combination wave
generator where its open circuit voltage is defined as
U
oc
, typically 6 kV 1.2/50 µs for the mains Class III test
and up to 4 kV 1.2/50 µs for signal/telecom Test
Category C.
With an impedance of 2
Ω
, the generator also
produces a peak short circuit current (sometimes
referred to as
I
sc
) at half the value of
U
oc
(3 kA 8/20 µs
for the mains Class III test and up to 2 kA 8/20 µs for
signal/telecom Test Category C).
With both voltage and current test waveforms, the
combined impulse test is designed to stress all
technologies used within SPDs.
Voltage Protection Level
U
p
is the key parameter that
characterizes the performance of the SPD in limiting
the transient overvoltage across its terminals. A low
protection level value (also known as let-through
voltage) is therefore particularly critical for the
effective protection and continued operation of
electronic equipment.
The peak voltage protection level
U
p
is declared when
the SPD is tested with its stated nominal discharge
current
I
n
(or the peak current (
I
peak
) of
I
imp
) and is also
declared when the SPD is subject to combined impulse
test (mains Class III test for Type 3 SPDs) as well as
data/telecom Test Categories C and B.
Common terminology and definitions
The following common terminologies, as recognised by BS EN/IEC 61643, are used throughout SPD specifications in
order to aid correct selection and are defined as follows: