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Damage from electrical transients, or surges, is one of the
leading causes of electrical equipment failure. An electrical transient
is a short duration, high-energy impulse that is imparted on the normal
electrical power system whenever there is a sudden change in the electrical
circuit. They can originate from a variety of sources, both internal and
external to a facility.
Not Just Lightning
The most obvious source is from lightning, but surges can
also come from normal utility switching operations, or unintentional grounding
of electrical conductors (such as when an overhead power line falls to
the ground). Surges may even come from within a building or facility from
such things as fax machines, copiers, air conditioners, elevators, motors/pumps,
or arc welders, to name a few. In each case, the normal electric circuit
is suddenly exposed to a large dose of energy that can adversely affect
the equipment being supplied power.
The following is a guideline on how to protect electrical
equipment from the devastating effects of high-energy surges. Surge protection
that is properly sized and installed is highly successful in preventing
equipment damage, especially for sensitive electronic equipment found
in most equipment today.
Grounding Is Fundamental
A surge protection device (SPD), also known as a transient
voltage surge suppressor (TVSS), is designed to divert high-current surges
to ground and bypass your equipment, thereby limiting the voltage that
is impressed on the equipment. For this reason, it is critical that your
facility have a good, low-resistance grounding system, with a single ground
reference point to which the grounds of all building systems are connected.
Without a proper grounding system, there is no way to protect against
surges. Consult with a licensed electrician to ensure that your electrical
distribution system is grounded in accordance with the National Electric
Code (NFPA 70).
Zones Of Protection
The best means of protecting your electrical equipment from
high-energy electrical surges is to install SPDs strategically throughout
your facility. Considering that surges can originate from both internal
and external sources, SPDs should be installed to provide maximum protection
regardless of the source location. For this reason, a "Zone of Protection"
approach is generally employed. The first level of defense is achieved
by installing an SPD on the main service entrance equipment (i.e., where
the utility power comes into the facility). This will provide protection
against high energy surges coming in from the outside, such as lightning
or utility transients.
However, the SPD installed at the service entrance will not
protect against internally generated surges. In addition, not all of the
energy from outside surges is dissipated to ground by the service entrance
device. For this reason, SPDs should be installed on all distribution
panels within a facility that supply power to critical equipment. Similarly,
a third zone of protection would be achieved by installing SPDs locally
for each piece of equipment being protected, such as computers or computer
controlled devices. Each zone of protection adds to the overall protection
of the facility as each helps to further reduce the voltage exposed to
the protected equipment.
Coordination of SPDs
The service entrance SPD provides the first line of defense
against electrical transients for a facility by diverting high-energy,
outside surges to ground. It also lowers the energy level of the surge
entering the facility to a level that can be handled by downstream devices
closer to the load. Therefore, proper coordination of SPDs is required
to avoid damaging SPDs installed on distribution panels or locally at
vulnerable equipment. If coordination is not achieved, excess energy from
propagating surges can cause damage to Zone 2 and Zone 3 SPDs and destroy
the equipment that you are trying to protect.
When selecting an SPD for a given application, there are several
considerations that must be made:
Application - Ensure that the SPD is designed for the zone
of protection for which it will be used. For example, an SPD at the
service entrance should be designed to handle the larger surges that
result from lightning or utility switching.
System voltage and configuration - SPDs are designed for
specific voltage levels and circuit configurations. For example, your
service entrance equipment may be supplied three phase power at 480/277
V in a four-wire wye connection, but a local computer is installed to
a single-phase, 120 V supply.
Let-through voltage - This is the voltage that the SPD
will allow the protected equipment to be exposed to. However, the potential
damage to equipment is dependent on how long the equipment is exposed
to this let-through voltage in relation to the equipment design. In
other words, equipment is generally designed to withstand a high voltage
for a very short period of time, and lower voltage surges for a longer
period of time. The Federal Information Processing Standards (FIPS)
publication "Guideline on Electrical Power for Automatic Data Processing
Installations" (FIPS Pub. DU294) provides details on the relationship
between clamping voltage, system voltage, and surge duration.
As an example, a transient on a 480 V line that lasts for 20 microseconds
can rise to almost 3400V without damaging equipment designed to this
guideline. But a surge around 2300 V could be sustained for 100 microseconds
without causing damage. Generally speaking, the lower the clamp voltage,
the better the protection.
Surge current - SPDs are rated to safely divert a given amount of surge
current without failing. This rating ranges from a few thousand amps
up to 400 kiloamperes (kA) or more. However, the average current of
a lightning strike is only approximately 20 kA., with the highest measured
currents being just over 200 kA. Lightning that strikes a power line
will travel in both directions, so only half the current travels toward
your facility. Along the way, some of the current may dissipate to ground
through utility equipment.
Therefore, the potential current at the service entrance from an average
lightning strike is somewhere around 10 kA. In addition, certain areas
of the country are more prone to lightning strikes than others. All
of these factors must be considered when deciding what size SPD is appropriate
for your application.
However, it is important to consider that an SPD rated at 20 kA may
be sufficient to protect against the average lightning strike and most
internally generated surges once, but an SPD that is rated 100 kA will
be able to handle additional surges without having to replace the arrester
Standards - All SPDs should be tested in accordance with ANSI/IEEE C62.41
and be listed to UL 1449 (2nd Edition) for safety.
Data Line Protection
Electrical transients are not confined to the electrical distribution
system. They can enter a facility through phone/fax lines, cable or satellite
systems, and local area networks (LAN). Therefore, in order to achieve
maximum protection from surge damage, SPDs should be installed on all
systems susceptible to electrical transients.
For maximum protection, SPDs should be installed as close
to the equipment being protected as possible. Cable lengths should be
as short and straight as possible to minimize the resistive path of the
circuit to ground. A solid connection to the system grounding conductor
is essential for proper operation of the SPDs. The surge protectors should
be equipped with indicators that show if the circuit is grounded and operating
properly, and the units installed so these indicators can be easily inspected.
All service entrance and distribution panel SPDs should only
be installed by a licensed electrician familiar with the equipment and
its use. In addition, Hartford Steam Boiler strongly recommends that a
professional engineer experienced with surge suppression technology be
retained to design the protection scheme for your facility to ensure all
SPDs are properly sized and coordinated.