Datasheet

II-2015, www.findernet.com
23
E
7P
SERIES
7P SERIES
Surge Protection Device (SPD)
SURGE VOLTAGE PROTECTORS
Surge voltage protectors (such as Finder’s Surge Protection Devices, SPD)
are intended to be installed in electrical systems, to protect people and
machines from surge voltages that can occur on the electrical supply line
and which would otherwise have disastrous consequences. These surge
voltages can be atmospheric (lightning) or can originate on the electrical
system due to, for example: the opening and closing of large loads, short
circuits, or the switching of large power factor correction capacitors. The
SPD can be described as a switch that is in parallel with the electrical
system’s supply line - which it is protecting. At the nominal network
voltage (e.g. 230 V) the SPD appears as an open switch, having a very
high impedance (almost infinite). But, under an overvoltage condition its
impedance rapidly falls to near 0Ω. This effectively applies a short circuit
across the supply lines and immediately “drains” the overvoltage to earth.
In this way the supply line is protected wherever an SPD is installed. When
the overvoltage has passed, the SPD impedance rises rapidly and resumes
the state of an open switch again.
Figure 1: Ideal operation of an SPD
SPD technologies
Finder surge voltage protectors use either varistors or spark gaps.
Varistor: This can be considered as a variable resistance that at nominal
voltage has a very high ohmic value. But the resistance rapidly falls to near
zero as the voltage surges. In this way the varistor applies a near short
circuit which clamps the surge voltage. The varistor is however subject
to progressive degradation due to the small leakage current that occurs
at the nominal voltage, and with the number of interventions. With every
overvoltage that occurs the leakage current rises and accelerates the end
of life for the device - which is ultimately indicated by the change from
green to red in the signal-window.
Spark gap: This comprises two electrodes separated by air, or a gas.
When a surge voltage occurs an electrical arc bridges the gap and a surge
current flows to limit the surge voltage to a low and constant level. The
arc extinguishes only when the surge current falls below about 10ampere.
The gas guarantees a constant level of breakdown voltage since the arc is
struck in a protected environment; not exposed to pressure or humidity
variations or impurities as would happen if it had occurred in air. There is
however, a delay before the device arcs and the surge current is diverted,
and this is dependent on the magnitude of the original voltage surge and
on its rate of rise. Therefore, the voltage protection level can vary, although
it is guaranteed to be less than U
p
.
Component Symbol
Leakage
current
Energy
dissipated
Response
time
Voltage/Current
characteristic
Ideal
Spark gap
Varistor Very Low Medium Fast
0 High Medium
0 High Fast
Figure 2: SPD component characteristics.
Installation (Overvoltage) categories
Choosing the SPD requires matching the Rated Impulse Voltage of the
SPD with that of the equipment to be protected. This in turn relates to the
Installation category (Overvoltage category). Installation categories are
described within IEC60664-1, which for a 230/400V installation prescribes
as follows:
- Installation category I: 1.5kV for “particularly sensitive” equipment (e.g.
electronic devices like PC or TV set);
- Installation category II: 2.5kV for “user” equipment subject to “normal”
impulse voltages (e.g. household electrical appliances, mobile items);
- Installation category III: 4 kV for equipment that are part of a fixed
installation (e.g. switchboards, switches)
- Installation category IV: 6 kV for equipment installed at or near the
origin of main incoming supply mains (e.g. energy meters).
Lightning Protection Zones and installation considerations
International standards refer to the various Lightning Protection Zones by
the letters LPZ followed by an appropriate number.
LPZ0A: An external area, where a direct lightning strike is possible
and where there is total exposure to the electromagnetic field
induced by the lightning.
LPZ0B: An external area, but below a lightning conductor providing
direct lightening strike protection. There remains total exposure
to the electromagnetic field.
LPZ1: Area within a building – therefore protected from direct lightning
strike. The electromagnetic field will be attenuated, depending
on the degree of shielding. This zone has to be protected by SPD
type 1 device(s) at its boundary with the LPZ0A or 0B zone.
LPZ2: An area, typically a room, where the lightning current has been
limited by preceding surge protectors. This zone has to be protected
by SPD type 2 device(s) at its boundary with the LPZ1 zone.
LPZ3: An area within a room where the lightning current has been
limited by preceding surge protectors (typically the wiring after a
socket or an area within a metal enclosure).
This zone has to be protected by SPD type 3 device(s) at its boundary
with the LPZ2 zone. On the following picture (Figure 3, representation
is not binding) it is shown that the transition from a protection zone to
the next is through the installtion of SPD. SPD Type 1 must be connected
upstream the system, at the point of delivery connection. As an alternative
it is possible to use SPD Type 1+2. The grounding conductor must have a
minimum section of 6mm
2
for SPD Type 1, of 4mm
2
for SPD Type 2, and
1.5mm
2
for SPD Type 3 (If the building has an LPS, reference should be
made to CEI81-10/4 for the correct dimension of the cable).
INSTALLATION CATEGORIES
(RATED IMPULSE VOLTAGE)
LPZ
SPD type 1
SPD type 2
SPD type 3
LPZ0
LPZ1
LPZ2
LPZ3
IV
(6kV)
III
(4kV)
II
(2.5kV)
I
(1.5kV)
e.g. electronic devices
like PC or TV set
e.g. household
electrical appliances,
mobile items
e.g. switchboards,
switches
e.g. energy meters
Figure 3: Typical relationship between Lightning Protection Zones, Installation
Categories and SPD types