The popularity of solid-state electronic devices has
grown dramatically over the last few decades. In fact, roughly half the
electrical power produced worldwide passes through such a device.
It’s easy to see why solid-state electronic devices
such as television, DVDs, microwave oven, stereo and computer, are so
popular. The devices are small in size, convenience to use, very precise
in their function, offering superior performance and exceptionally durable
and long lasting.
However these electronic devices have a downside. To
provide the precise, dependable function these devices require an equally
dependable source of power.Performance could be hampered or even stopped
or in the case of a major voltage surge (100V-20KV), these devices could
be damaged or even destroyed.
The function of the surge protector is to stop (or at
least limit) the effects of less than perfect power quality on electronic
devices. Surge protector is a cost-effective solution to prevent downtime
and equipment damage.
An electrical surge or transient is a random, high
energy, short duration electrical disturbance which also define as voltage
impulse and spike.
Sources of such surges include:
- Lightening
- Switching loads
- Short circuits
- Variable speed drive operation
- Imaging equipment operation (photocopier and scanner)
- Arc welding
- Lighting dimmer
A surge can travel on electrical, telephone and
coaxial cable lines entering a building. It is too fast to stop by circuit
breakers or fuses and it can damage solid state devices and corrupt
microprocessor data.
The use of surge protector device helps to alleviate
some of the problems caused by electrical surges.
Short period of undervoltage and overvoltage conditions are called sag and swell in electrical line voltage. They can be caused by one or multiple large loads turning on or off at the same time. The duration of sag and swell is generally between 8.3 millisecond (½ cycle) and one minute. They can cause the computers to shut down or lock up in a program, motors to stall, and contactors and relay to open. The use of voltage regulators and power line conditioners can solve these problems.
Noise distortion takes the form of unwanted electrical signals present on the steady-state voltage waveform in the shape of low voltage high frequency signal. Radio Frequency Interface (RFI) produced mainly by communication system components, can contribute to system noise. Have you ever experienced static on your TV caused by a hair dryer or a nearby CB radio? These are example of RFI.
Another form of similar noise is brought about by poor grounding systems. Our audiophile customers should be familiar with this interface.
The most destructive power disturbance of all is lightning. First, let’s consider a few statistics from the National Lightning Safety Institute (NLSI):
- At any giving time, over 1,000 thunderstorms are in progress worldwide.
- Lightning strikes the ground somewhere on our planet over 100 times every second.
- Tall structures, such as audio towers and office high-rises are struck by lightning 5 to 10 times a year. Smaller buildings can expect to be struck once per year.
- Lightning strike in the United States cost over $2 billion in annual economy losses. Mainly through computer damage and data loss. This figure climbing rapidly as microchip-driven devices continue to gain popularity.
Lightning is a more serious problem than most people think. In fact, NLSI states that lightning is an under-rated hazard.
If a building is hit directly, an impulse current measured in thousand of amperes can find its way into the facility’s electrical power, data, and telephone lines.
There are two types of surge: The combination wave
and the ring wave
The combination wave waveform is a unipolar pulse,
similar to a lighting strike. The rise time of this current wave is 8
microsecond.
The ring waveform is an oscillating waveform that
most often occurs inside a premise.
The amplitude and available energy of the standard
waveforms are dependent upon the location within the house. For this
reason, the standard classifies the location in three categories:
- Category C: outside electrical service up to the electrical panel
- Category B: electrical panel, short distance branch circuit, feeders to heavy appliances with short connection to service entrance.
- Category A: outlets and long branch circuits; all outlet that are more than 30 feet from category B and more than 60 feet from category C
|
Category |
Voltage(V) |
Current (A) |
|
|
|
Ring Wave
0.5 nS- 100KHz |
Combination Wave
(Impulse)
8-20 nS |
|
A3 |
6,000 |
200 |
N/A |
|
B3 |
6,000 |
500 |
3,000 |
|
C3 |
20,000 |
N/A |
10,000 |
The design goal of a surge protective device is to
divert as much of a transient power disturbance away from the load as
possible. To accomplish this, a Surge Protective Device redirects the
surge and transient voltage to ground.
The system ground provides path for the surge to
follow to the earth grounding connection, preventing damage to the
electrical equipments. If the ground is not solid, the surge will go
through alternative paths for example electronic equipment, leading to
equipment damage. Roughly 80% of power quality issues are tied to
grounding problem.
If a surge strip (a grouping of few receptacles
connected to an extension cord, with an integrated surge protective
device) is all that stands between your expensive electronics (stereo, TV,
computer , etc.) and a lightning strike, your investment is not as safe as
you think.
Surge strips alone can’t provide sufficient
protection. The typical surge strip provides a current rating of 2,000 to
6,000 amps. The maximum lightening surge is defined as 20,000 volt and
10,000 amps. When the lightning strike overwhelms that surge strip, you
can guess where the rest of that surge goes.
IEEE recommends that SPDs be coordinated in a staged
or cascaded approach. The First SPD which we call whole house surge
protector should be placed at main electrical panel (service entrance) to
reduce the voltage surge to the acceptable level for any downstream
devices including surge strips. To deal with any residual voltage, a
second SPD in the form of surge strip should be installed just ahead of
critical loads, such as computers, sound systems, TV, etc. This will
reduce a 20 KV lightning surge to well under 330 volts peak.
Let-Through voltage or clamping voltage is the amount
of voltage that is not suppressed by the surge protective device and
passes through to the load. Lower Let-Through voltage offers better
protection for downstream load.
Surge current per phase: For houses 120 KA or more
per phase for panel boards or other location. Surge current capacity is an
indication of a SPDs life expectancy. The higher SPDs surge current
capacity rating, the greater its life expectancy. We will look at this
shortly.
Let Through Voltage: Performance should be specified
based on three standard IEEE test wave form (category C3 and B3
Combination waves, and B3 Ringtone). Lower let-through voltage provides
better protection.
Effective filter: Noise attenuation at 100 KHz should
exceed 50 dB (L-N modes)
IEEE,IEC and NEMA do not recommend using Joule
ratings when specifying or comparing surge suppressors because they can
provide misleading and conflicting information. For example on a 120 volt
system, a 150 volt or 175 Volt MOV could be used. Even though 175 Volt MOV
Has a higher joule rating, the 150 Volt has a much lower let through
voltage. Joule rating is a function of let-through voltage, surge current,
and surge duration. Each manufacturer may use a different standard surge
wave when publishing surge Joule rating.
Industry standard publish surge current per-phase, by
summing surge current per modes: Line-Neutral mode and Line-Ground mode in
a wye system, or Line-Line mode and Line-Ground mode in a Delta system.
Surge current capacity should be stated on a per-phase and per-mode basis
when specifying an SPD for a given application.
Based on available research, the maximum amplitude of
a lighting-related surge on a facility’s service entrance is a 20 KV, 10
KA combination wave. Above this amount, the voltage will exceed basic
insulation rating, causing arcing in the conductors and/or the
distribution system.
But the answer is life expectancy. A service entrance
SPD will experience thousand of various magnitudes. Based on statistical
data, a properly constructed SPD with a 250 KA per phase surge current
rating will have a life expectancy of 25 years in a high exposure
location. A 400 KA/phase SPD has 500-year life expectancy.
The failure rate of an SPD is 0.1%, very low! Should
a SPD fail, it is likely the result of excessive overvoltage (swell, i.e.
the nominal 120 VAC line exceed 180 VAC for many cycles) duo to a fault on
the utility power line. Should this rare event occur, call the utility to
investigate the problem.
Metal Oxide Varistor is a reliable and proven
technology available for reducing a transient voltage. In AC power
applications, over 99% of SPDs incorporate MOV technology. Under normal
operating conditions, the MOV is a high impedance component which has a
small leakage current passing through it. But, when subjected to a voltage
surge of, Say, 125% system nominal voltage, the MOV reacts in nanoseconds,
becoming a low impedance path to divert the surge away from the load.
Manufacturers are not required to have their units
independently tested to their published surge current capacity rating.
Most published ratings are theoretical. They are calculated by summing the
individual MOV capabilities. For example, a manufacturer may claim a
rating of 100 KA, but due to the poor construction integrity, the unit may
be unable to share current equally to each MOV. Without equal current
sharing, the expected life expectancy cannot be met.
Installation is the most important factor in
determining the effectiveness of a particular SPD. Installation wire
length reduces the performance of any surge suppressor. As a rule of
thumb, each inch of installation wire length adds between 15 to 25 volts
to the let-through voltage. Because surge occur at high frequency (about
100 KHz), the wire from the bus bar to the suppression element creates
impedance in the surge path.
Published let-through voltage ratings cover the
device or module only. These ratings do not include installation wire
length, which is dependent on the electrician installing the unit.
For example, consider an SPD with a 500 Volt rating.
This is the true rating only if the SPD is integrated into the panelboard
it is protected. If it is connected to a panelboard with 14 inches of #14
wire, it allows approximately 300 Volts to be added to the let through
voltage. The true let through voltage at the bus bar is 800 Volts.
Surges also enter a home through antenna cable,
coaxial cable and phone line. Surge Protective Devices (SPDs) should be
installed at the service entrance on all cable conductors.
If you have any special case or need further
information please contact us at 24 Hour Electrician.