SIEMENS September, 2003 T e c h n i c a l . J o u r n a l . I 2 t vs. I 4 t - What's the difference? Prepared by Ray Clark, Senior Application Engineer Everyone knows that one of the purposes of a circuit breaker is to provide overload protection of conductors. Most people know that this relates to the thermal capacity of the conductors, i.e. the circuit breaker must trip before the conductors get hot enough to be damaged. Many people know that the term "I 2 t", or ampere-squared-seconds, is an expression of the energy, or heat, generated by electrical current. IEEE standard 1015-1997, "IEEE Recommended Practice for Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems", paragraph 2.1.19 defines I 2 t as: "An expression related to the energy available as a result of current flow, meaningful only for adiabatic conditions. With respect to circuit breakers, the expression refers to the I 2 t between the initiation of fault current and the clearing of the circuit. …" The term "I 2 t" is most often seen in conjunction with electronic trip circuit breakers, in a lot of cases it's marked right on the front of the trip unit itself. Several trip functions found in modern digital electronic trip units determine tripping based on calculation of I 2 t. Generally offered are long time delay, short time delay, and ground fault delay. The I 4 t function is of course based on a different calculation and provides different trip timing for a given current, but is simply an extension of I 2 t. The generally slower trip time might provide a benefit in getting a particular load online, but doesn't in itself usually provide an advantage. The I 4 t function does offer an advantage in another area. Both the I 2 t and I 4 t functions are useful in the area of selective trip coordination. The log-log plot of an I 2 t delay function more closely matches the sloping curve shape of a thermal-magnetic circuit breaker or a fuse. It therefore provides a better "fit" in many instances. The overlap of the trip curves may be eliminated and selective trip coordination achieved in that region. However, as the ampacity of two overcurrent devices grows closer, the chances of an overlap grow greater. The graph in Figure 1 shows just such an overlap between a fuse and a downstream circuit breaker with an I 2 t long time delay. In Figure 2 the advantage of the I 4 t long time delay is clear, even with all other settings remaining the same, the miss-coordination is eliminated. Four of the WL trip units offer both I 2 t and I 4 t long time delay functions to allow the user greater flexibility in selective trip coordination. 1
. J o u r n a l . I2t vs. I4t - What's the difference?Prepared
by Ray Clark, Senior Application Engineer Everyone knows that one
of the purposes of a circuit breaker is to provide overload
protection of conductors. Most people know that this relates to the
thermal capacity of the conductors, i.e. the circuit breaker must
trip before the conductors get hot enough to be damaged. Many
people know that the term "I2t", or ampere-squared-seconds, is an
expression of the energy, or heat, generated by electrical current.
IEEE standard 1015-1997, "IEEE Recommended Practice for Applying
Low-Voltage Circuit Breakers Used in Industrial and Commercial
Power Systems", paragraph 2.1.19 defines I2t as: "An expression
related to the energy available as a result of current flow,
meaningful only for adiabatic conditions. With respect to circuit
breakers, the expression refers to the I2t between the initiation
of fault
current and the clearing of the circuit. " The term "I2t" is
most often seen in conjunction with electronic trip circuit
breakers, in a lot of cases it's marked right on the front of the
trip unit itself. Several trip functions found in modern digital
electronic trip units determine tripping based on calculation of
I2t. Generally offered are long time delay, short time delay, and
ground fault delay. The I4t function is of course based on a
different calculation and provides different trip timing for a
given current, but is simply an extension of I2t. The generally
slower trip time might provide a benefit in getting a particular
load online, but doesn't in itself usually provide an advantage.
The I4t function does offer an advantage in another area. Both the
I2t and I4t functions are useful in the area of selective trip
coordination.
The log-log plot of an I2t delay function more closely matches
the sloping curve shape of a thermal-magnetic circuit breaker or a
fuse. It therefore provides a better "fit" in many instances. The
overlap of the trip curves may be eliminated and selective trip
coordination achieved in that region. However, as the ampacity of
two overcurrent devices grows closer, the chances of an overlap
grow greater. The graph in Figure 1 shows just such an overlap
between a fuse and a downstream circuit breaker with an I2t long
time delay. In Figure 2 the advantage of the I4t long time delay is
clear, even with all other settings remaining the same, the
miss-coordination is eliminated. Four of the WL trip units offer
both I2t and I4t long time delay functions to allow the user
greater flexibility in selective trip coordination.
1
Technical I2t vs. I4t - What's the difference? September, 2003 .
Journal . . .
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