Download (Joule) Ratings

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Energy (Joule) Ratings
Transient
Protection
System
There are many different components used to control transient voltages. In various pieces of equipment, silicon avalanche
diodes (SAD), metal oxide varistors (MOV) and gas tubes are utilized. The failure mode of gas tubes is primarily due to
excessive current. This leads to the units being rated by maximum current it can safely conduct. MOV and SAD components
typically fail due to excessive energy and most components are rated in maximum energy (Joules) or power (Watts), which
can be dissipated.
Power is work (energy) per unit time. By changing the time frame, the amount of power will vary. This is due to the one time
nature of component rating. Energy will generally remain constant over a wide time frame. Both maximum power and
maximum energy are calculated from measured current and voltage waveforms.
As test pulse shapes change, so do the values for power and energy. Some of the test pulses used include an 8x20 ms wave
shape, 10 x 1000 ms wave shape, 2 ms square wave, and a 5 x 50 ms wave shape. Each test pulse will yield a different value
for the same component. So when comparing specifications, it is important to use the same units and the same test pulse.
Seldom is this the case.
The method of determining energy is to first define a failure condition then determine which test pulse to use and finally to
simultaneously record the voltage wave and the current wave. The voltage multiplied by the current will give the power
curve. This integrated with respect to time yields the energy value. As the test current is increased, the energy will increase.
At some point, the device will fall into the fail condition. The value previously recorded will be the maximum energy
measured in Joules.
As one can see, there are many variables that may influence the net result or value. This all aside from the fact that the
majority of surge suppressors currently available in the market place incorporate primarily nonlinear suppression
components. This nonlinear characteristic further complicates matters of true evaluation and test. The following is excerpt
from ANSI/IEEE C62.41-1991:
6.5.5.
Energy Delivery Capability. Recent surveys have addressed the issue of energy delivery capability in various
manners. As discussed in section 7, the significant parameter is not the “energy contained” in the surge but the
actual energy that can be deposited in a surge absorbing device. One survey author (Goedbloed, 1987[B14])
proposed an “energy measure” parameter, defined as the product of the voltage square by the time duration of
the voltage. This approach is justifiable for a resistive load, where the power dissipated in the resistor is V2/R.
For a nonlinear surge protective device, the relationship is not so simple. Furthermore, the concept of recording
the “energy measure” may promote the arbitrary reporting of “surge energy” by assuming a value for the
impedance and then quoting results in joules.”
From this IEEE document, one can conclude that energy delivery capability of a surge can be difficult to calculate when that
surge is absorbed or shunted by a nonlinear device. Why then is it necessary for SPD manufacturers to print energy dissipation
figures in joules? Because they are utilizing linear devices; or are they? Typically, manufacturers that don’t advertise joule
ratings are not utilizing this technology in their device. However, there are some manufacturers that do advertise joule
ratings and the values they print are a calculation of the energy dissipation capability of each individual discrete component
as listed by the manufacturer of that discrete component. These manufacturers often publish these joule rating values to
disguise or distract from other less flattering values. Seldom do reputable manufacturers utilizing nonlinear components
advertise joule ratings. This is primarily due to the fact that nonlinear components have a variable joule rating value that is
influenced by the characteristics of a particular surge event. The manufacturers that do utilize nonlinear components and
advertise joule ratings, tend to calculate their joule rating by virtue of summing the highest value in the scale of the joule
values for that particular nonlinear component. This is truly a deceptive maneuver on their behalf. You will notice the variable
values listed below.
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Transient energy ratings are a source of some confusion in the industry. The rating, in joules (watt-second), is the maximum
allowable energy for a single impulse of 10/1000 ms current waveform with continuous voltage applied. For cases where the
transient is from a source external to the equipment, an approximation technique can be used to estimate the energy of the
transient absorbed by (in the case of an MOV) or shunted by (in the case of a gas tube) the suppression device. The method
requires finding the transient current and voltage through the device. To determine the energy absorbed (or shunted), the
following equation applies:
E=
∫V c(t ) I (t )∆ t
t
0
= K V cIt
Where I is the peak current applied, Vc is the clamp voltage which results, t is the impulse duration and K is a constant1 . From
this it can be seen that energy dissipation is directly proportional to the clamp voltage and the duration of the impulse. The
higher the impulse duration, the clamp voltage, or the peak impulse current, the higher the energy rating of the device. There
is a trade-off here between low clamp voltage and high joule ratings. Since the object in surge suppression is to minimize the
voltage spike that protected equipment experiences, it is much more advisable to focus on the clamp voltage as the primary
quantity to specify. Most SPD units manufactured by reputable manufacturers utilize MOV technology. The larger the MOV,
the larger its joule rating (the factor K above is a function of MOV size). On page 4-7 of the Harris manual, Figure 4-9 indicates
that a 20mm MOV has a joule rating of 70 to 250 while a 40 mm MOV is rated at 270 to 1050 joules. All Siemens products
utilize the larger 40mm MOVs and are thus able to provide higher peak surge ratings with lower clamping voltages.
Whether or not the joule rating is an effective means of measuring a surge device’s energy absorption capability is up for
personal speculation and evaluation, but one must remember that joule rating is not criteria defined by NEMA LS1 or ANSI/
IEEE as criteria deemed essential for a surge suppressor. Energy absorption characteristics are best defined and evaluated
using “Maximum Surge Current” capability. The parameters surrounding this performance criteria are far more defined.
1
Reference: Transient Voltage Suppression Devices, Harris Corporation, 1992, USA, p. 4-3.
Here is an “intriguing” quote from a manufacturer of SPD devices that advertises Joule Ratings in their literature:
Joule Rating. For those who would apply a Joule
Rating to a surge protector, we advise caution. While
a high Joule Rating may describe the ruggedness
of a protector and its ability to survive high energy
surges, a high Joule Rating may not adequately
describe the protective capability of the protector.
A Joule is a Watt-second. A Watt-second is the
product of the voltage in volts, the current in amperes,
and the time in seconds. In the test situations prescribed by various ANSI/IEEE standards, the current
amplitudes and the waveforms (i.e. time) are definitively
specified. Only the foltage drip across the surge
protector is the unknown. This voltage drop is referred
to as the suppression voltage of the protector. The
lower the suppression voltage, the lower the voltage
appearing across the circuit or device being protected.
Therefore the lower the suppression voltage, the better
the protection; and the lower the suppression voltage,
the lower the energy dissipation of the surge protector
in Joules.
Remember, the theoretical ultimate in proteciton
would be a surge protector whose suppression voltage
is zero relative to the normal AC system voltage. This
would result in a Joule Rating of zero. Although
Specifications Subject to Change Without Notice.
Manufactured in U.S.A.
20,000 amperes as ea
does in its first.
It should be noted th
where surge currents v
ampere peak specified
installations, a protector
current capability would
3. Suppression Voltage
Voltage perhaps best d
of the surge protector.
Of primary importance
the diversion of the high
pass through the protec
surge protector must have
tive to that of the circuit and
is being used to protect
protector have a low ener
load the transient source
across both the protector
The suppression voltage
low-impedance charact
ANSI/IEEE standard
for use at service entrance
surge current capability
JOSLY
The article then goes on to say:
For those who are still intrugued by Joule Ratings,
we include them in our specifications, but only to
indicate the robustness of our surge protectors,
a quality that is better defined by the maximum
surge current capability.
Protection Considerations: There are four primary
considerations when choosing a surge or transient
protection device. They are:
1. Maximum Continuous Operating Voltage
2. Maximum Surge Current
3. Suppression Voltage
4. Surge Life
Without values for each of these parameters, it is
impossible to compare surge protector capabilities.
It is divvicult to put these characteristics in order of importance as they are all very important wouldnít you say.
ELECTRONIC SYSTEMS
A JOSLYN CO
As one can see, Joule Ratings can be rather deceiving or erroneous information. We would caution the use of this data as a
means of evaluating surge suppressors and their performance.
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