ERICO Practical Guide to Electrical Grounding - Manual (Page 122)

115 Chapter 7: Application Of Surge Protection Devices
SURGE PROTECTION
Good grounding without good surge protection may not be
totally effective in protecting equipment and data. Surge
protection devices (SPDs) are usually needed. These
devices are proven and inexpensive - the best life insurance
your money can buy. But SPDs must be selected and
installed properly, otherwise they are not very effective.
Another term for SPDs is Transient Voltage Surge
Protectors (TVSS) but we will use the term SPD here.
The electric power company uses surge protection devices
called lightning arresters to protect its own facilities and
equipment. The building owner or tenant must also supply
surge protection devices to protect his electronically
controlled apparatus including computers, variable
frequency drives, PLCs, etc. Residences often have
computers, electronically controlled heating and cooling
systems and appliances which should be protected. Sources
of transients include induced or conducted manmade
transients which arise on incoming power lines and inside
the facility, as well as from lightning. In commercial and
industrial facilities most transients arise from within the
facility. The equipment itself may generate transients.
SPDs are manufactured using a variety of technologies.
These choices all provide advantages and disadvantages.
By far the most widely used technology is the Metal Oxide
Varistor (MOV) which consists of a pellet or block of
specially prepared zinc oxide with “impurities” added to
provide the desired voltage limiting characteristics. MOVs
are fast and give excellent protection at low cost in most
situations.
MOVs “clip” the voltage transient at a known level which
should be above the maximum possible steady state value
of the peak line voltage. A protective level of 300 volts or
even 400 volts is not unreasonable for most 120 volt
applications. Many specifiers try to “improve” the
protection level by overspecifying MOVs. Not only is this
unnecessary, it reduces the reliability of the overall system.
The SPD will have a rating called the MCOV (Maximum
Continuous Operating Voltage). This is the maximum value
of continuous rms voltage which the SPD can reliably
withstand.
Because MOVs have limited capability to absorb energy, a
standard has been proposed based on extensive studies by
the National Institute of Science and Technology (NIST)
and others, to assist the specifier. Many of these findings
are incorporated in ANSI/IEEE Std. C62.41, UL1449 and
corresponding CSA Standards.
Some suppliers of MOVs promise speed of operation of a
few nanoseconds. In industrial systems, most transients of
any significance are much slower. Indeed, rarely is the
response time of the SPD component itself of significience
because the inductance of the interconnecting conductor
tends to slow the transient risetime. The arrangement and
length of the SPD wiring is important. Devices tested to UL
1449 will be assigned a Suppressed Voltage Rating (SVR)
which indicates the clamping voltage of the device when
tested with a specific impulse. The SVR is an important
figure for the SPD.
Branch circuits feeding valuable equipment including
process control devices, computers and PLCs need their
own SPDs. These must be carefully sized for the voltage
and energy levels to which they may be subjected. The
energy rating of the branch circuit protection SPD can be
lower than that of the service entrance protector. Its voltage
rating is selected to be somewhat closer to the actual branch
circuit voltage to provide better protection. SPDs are also
needed at the point of utilization, or, better yet, inside each
piece of equipment. The SPDs need to be coordinated so
the larger (and more costly) service entrance SPDs absorb
most of the transient energy. This would allow the SPD at
the equipment utilization location point to minimize
voltage rise to a more acceptable value.
Connection of each SPD is also critical to their proper
performance. Short leads are needed on either side of the
SPD to minimize voltage drop from high frequency
transients. It is very possible that one additional foot of
conductor connected to the SPD may add over 1000 volts
to the voltage imposed on equipment. Of course, all
connections must be clean and tight. One element of these
connections is the fact that when they conduct surge
current, they raise the voltage along the ground conductor
to which they are connected. This voltage rise may be large
enough to upset the same or other equipment on the same
line. The solution to this unavoidable situation is to assure
low impedance in the grounds, especially those associated
with interconnected equipment. Signal Reference Grids are
one form of desirable solution as described in IEEE Std
1100-1992 “IEEE Recommended Practice for Powering
and Grounding Sensitive Electronic Equipment”.
The supplier of SPDs should be able to supply proof of
conformance to ANSI/IEEE Std. C62.41- 1991 or latest
revision, as well as UL1449 or appropriate CSA standards.
IEEE Std C62.41-1991 defines three location categories
with SPDs designed separately for each location category.
Location Category C is the incoming service to the
building and is the location where the highest energy is
Grounding Book 4/14/99 10/5/99 6:02 PM Page 115 (Black plate)