Download A Practical Guide to Selecting Surge Protection Devices ERICO

37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
ABN 70 078495646
A Practical Guide to Selecting Surge Protection Devices
By: Rohit Narayan, BE Electrical, MIEAust, CP Eng., MBA Tech Mgmt
ERICO Australia www.erico.com
The use of surge protection devices, or SPD’s, in low vo ltage (240/415V) power systems,
is a relatively new concept, compared to the use of other inherent components, like circuit
breakers, fuses, links and isolators.
The selection of appropriate surge protection devices is somewhat of a mystery in the
industry, for this and other reasons. In Europe and the USA there are comprehensive
standards, written on the testing, selection and application of SPD’s. Even then, there is
no clear protocol or common understanding of what is a good surge protection scheme.
There are various performance measurements, that are written down in the specification
sheets or nameplates of SPD’s.
This article attempts to discuss and explain the 4 key performance yardsticks that one can
use, in making the appropriate selection of SPD’s. These yardsticks are well defined and
discussed by IEC61643 standards and they are:
•
•
•
•
Maximum Discharge Current, or Imax
Nominal Discharge Current or In
Voltage Protection Level, or Up
Maximum Continuous Operating Voltage, or Uc as defined by IEC standards
Let us now discuss what the key performance yardsticks tell us about the performance of
the SPD in more detail.
1) Maximum Discharge Current, or Imax
The Imax gives an indication of the amount of surge energy the SPD will be able to handle
without getting damaged.
The Imax , is the maximum single shot current, the SPD can handle.
ABN 70 078495646
37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
The current and the voltage of the wave shapes at which this single shot is applied, are
defined by IEC standards.
The graph in Figure 1 is taken out of AS1768 2003 Interim Standard and provides a
depiction of what these wave shapes look like.
Figure 1 Standard Voltage and Current Wave Shapes
2) Nominal Discharge Current or In
In is an indication of how long the device will last in the power system.
It is worth highlighting a common misunderstanding, that SPD’s are only good for one shot
of a lightning surge. This is not true and there are many devices available that may
withstand thousands if not tens of thousands of surges.
The IEC standards require SPD’s tested for common power system applications, to
withstand 15 impulses at In followed by 10%, 25%, 50%, 75% and 100% of Imax . In is a
multiple shot rating of a SPD.
Obviously a SPD, with a higher In rating, will withstand more surges and last longer.
37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
ABN 70 078495646
3) Voltage Protection Level, or Up
Typically, SPD’s are connected between the phase and the earth or the neutral in the case
of MEN systems. The way a SPD works, is that remains open circuit at nominal voltages,
but if the voltage exceeds its clamp voltage, the SPD will temporarily short out to ground.
This enables the excess energy to be diverted to earth. This is perhaps the reason that
SPD’s are also commonly referred as surge diverters.
Figure 2 shows typical wiring of SPD’s.
Supply Transformer
Surge Diverter
63 Amps
16 mm2
SPD
Neutral
CB or Fuse
Earth
Various Loads
Figure 2 – Typical Wiring of SPD’s
37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
ABN 70 078495646
The Up characterises the performance of a SPD in limiting the voltage. The Up indicates
how well the SPD, clamps an applied surge and hence a SPD with a lower Up, is a better
device in terms of limiting the voltage across an equipment. The graph in Figure 3 below
shows what the Up is with respect to the applied standard voltage wave shape and the
clamp voltage.
Peak Voltage is
approximately
6000V
Voltage
Applied Voltage
Voltage
Protection
Level, Up
Clamp V
Voltage After Clamping
by SPD
Time in micro seconds
Figure 3 – Voltage Protection Level, Up
4) Maximum Continuous Operating Voltage, or Uc
SPD’s are voltage-limiting devices and it is important to select a SPD that will not attempt
to clamp slight over voltages at 50 Hz. Uc is a guide to how rugged the SPD is against
over voltages. If the SPD attempts to clamps the voltage continuously, then this can either
result in damage to the SPD or even a fire hazard if the SPD get hot.
37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
ABN 70 078495646
Example: Comparing Performance Yardsticks of Two Products.
This example demonstrates, that two SPD’s which have identical Imax ratings may have
starkly different In, Up and Uc. The CRITEC TDS MT 277 is the better choice in this
example.
CRITEC TDS MT 277
CRITEC DSD1100
Imax = 100 kA
In = 80 kA
Up= 750 V at 3 kA(8/20µs)
Up=1000V at 20 kA(8/20µs)
Uc= 480V
Imax = 100 kA
In = 50 kA
Up= 850 V at 3 kA(8/20µs)
Up=1700V at 50 kA(8/20µs)
Uc= 275V
Figure 4 : Comparing Two SPD’s
It is worth pointing out that there are several schemes that can be used for choosing
installation locations of SPD’s. The choice of the scheme depends on cost, the sensitivity
of the equipment being protected, the frequency of the occurrence of surges, the
importance of the systems or the processes being protected.
For example, a simple scheme would have a SPD with low Up and a high Imax and In at
the Main Switchboard an no subsequent downstream protection.
In a larger installation, there may be a need to install a SPD at the main switch board, as
coarse primary protection and SPD’s on distribution boards as finer secondary
protection.
ABN 70 078495646
37 Railway Road
Blackburn South, VIC 3130
Ph: (03) 9894-2677
Fax: (03) 9894-3216
Email: [email protected]
Web: www.erico.com
In critical applications, surge reduction filters or SRF ’s are used as finer protection. The
yardsticks described above, that is, Imax, In, Uc and Up can still be used to define
performance of surge reduction filters, SRF’s. But other than lowering the voltage to which
the equipment is exposed, the SRF’s also reduces the voltage rise time or dv/dt. It is
widely recognised that electronic equipment is at danger of being damaged, both, from
large amplitudes and high rise time associated with power surges. While surge diverters
or SPD’s take care of the amplitude factor only, the SRF’s take care of the amplitude and
the dv/dt factors both. Figure 5 below, explains the performance of CRITEC TSG SRF.
Figure 5 : Operation
of CRITEC TSG SRF
The available literature in standards and elsewhere on SPD’s is extensive. Hopefully this
paper provides a simple and practical understanding of some key performance yardsticks
that can be used to select SPD’s and SRF’s.