Properly engineered and installed selective coordination between LV circuit breakers

Time-Current Characteristic Curves

It’s extremely important to work on selective coordination of circuit breakers in the design phase of a system. After switchboards, distribution panels, MCCs, lighting switchboards, etc. are installed, there typically is little that can be done to retroactively “fix” a system that is not selectively coordinated.

While it’s very important, it is not enough to select circuit breakers based solely on their ability to carry the system load current and interrupt the maximum fault current at their respective points of application.

It is important to note that the type of overcurrent protective devices and ratings (or settings) selected determine if a system is selectively coordinated.

Power blackouts cannot be tolerated.

Contents:

1. Circuit Breaker Curves
   1. Overload Region
   2. Instantaneous Region
   3. Interrupting Rating
2. Achieving Selective Coordination with Low Voltage Circuit Breakers
   1. Two Instantaneous Trip Circuit Breakers
   2. Interpreting Circuit Breaker Curves for Selective Coordination
   3. Analysis for branch circuit fault
   4. Analysis for feeder circuit fault
   5. Conclusion for coordination analysis

1. Circuit Breaker Curves

When using molded case circuit breakers of this type, there are three basic curve considerations that must be understood (see Figure 1). These are overload region, instantaneous region with unlatching and interrupting rating.

1.1 Overload Region

Overloads typically can be tolerated by the circuit components for relatively longer times than faults and therefore, the opening times are in the range of seconds and minutes.

As can be seen, the overload region has a tolerance band, which means the breaker should open within that area for a particular overload current.

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1.2 Instantaneous Region

A circuit breaker will open as quickly as possible. The instantaneous trip (IT) setting indicates the multiple of the full load rating at which the circuit breaker starts to operate in its instantaneous region.

The instantaneous region is represented in Figure 1, and for this example, is shown to be adjustable from 5× to 10× the breaker amp rating.

When the breaker senses an overcurrent in the instantaneous region, it releases the latch which holds the contacts closed (unlatches). Unlatching permits the contact parting process to start.

The unlatching time is represented by the curve labeled “average unlatching times for instantaneous tripping” (this is the continuation of the instantaneous trip curve below 0.01 second). This is important when evaluating corrdination of line side breakers to load side breakers.

The manufacturer of the circuit breaker in Figure 1 also published a table of unlatching times for various currents (upper right). Unlatching frees or releases the spring loaded contacts to start the process of parting.

After unlatching, the overcurrent is not cleared until the breaker contacts are mechanically separated and the arc is extinguished (represented in Figure 1 as the maximum interrupting time).

Consequently, there is a range of time from unlatching to interruption as is indicated by the band between the unlatching time curve and the maximum interrupting time curve.

This range of time affects the ability of circuit breakers with instantaneous trips to selectively coordinate when the overcurrent magnitude is in the instantaneous trip range.

Two instantaneous trip settings for a 400A breaker are shown in Figure 1.

The dashed portion represents the same 400A breaker with an IT = 10x, or 10 times 400A = 4000A. At this setting the overload trip will operate up to approximately 4000 amps (±10%).

Overcurrents greater than 4000A (±10%) would be sensed by the instantaneous setting. The ±25% and ±10% band mentioned in this paragraph represents a tolerance. This tolerance can vary by circuit breaker manufacturer and type.

Many of the lower amp rated circuit breakers (100A and 150A frame CBs) have non-adjustable or fixed instantaneous trip settings. For larger molded case, insulated case and power breakers the instantaneous trip setting can usually be adjusted by an external dial.

The IT of a circuit breaker is typically set at its lowest setting when shipped from the factory.

Note that most published circuit breaker time-current curves show the vertical time axis from 0.01 second up to about 100 or 1000 seconds. The published curves do not normally provide the instantaneous unlatching characteristic.

However, if a circuit breaker has an instantaneous trip, it has unlatching times usually less than 0.01 second.

Typically, molded case circuit breakers and insulated case circuit breakers that have short time-delay settings have an instantaneous trip override. This means at some fault current level, the instantaneous trip operates to protect the circuit breaker.

Low voltage power circuit breakers can be specified with a short time-delay setting which does not inherently incorporate an instantaneous trip override.

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1.3 Interrupting Rating

The interrupting rating is represented on the drawing by a vertical line at the right end of the curve. The interrupting rating for circuit breakers varies based on the voltage level.

See the interrupting rating table in Figure 1 which lists the interrupting ratings for this specific circuit breaker.

In Figure 1, the circuit breaker interrupting rating at 480 volts is 30,000 amps.

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2. Achieving Selective Coordination with LV Circuit Breakers

To achieve selective coordination with low voltage circuit breakers, the general rule is that no overlap of time-current curves (including the unlatching time) is permitted up to the available short-circuit current.

The ability of circuit breakers to achieve coordination depends upon the type of circuit breakers selected; amp ratings, settings and options of the circuit breakers, and the available short-circuit currents.

The type of circuit breaker selected could be one of three types:

1. Circuit breakers with instantaneous trips;
2. Circuit breakers with short time-delay but incorporating instantaneous overrides; or
3. Circuit breakers with short time-delays (no instantaneous override).

In the following paragraphs, various alternative circuit breaker schemes will be discussed in relation to assessing for selective coordination.

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2.1 Two Instantaneous Trip Circuit Breakers

Figure 2 illustrates a 90A circuit breaker and an upstream 400A circuit breaker having an instantaneous trip setting of 5× (5 times 400A = 2000A).

The minimum instantaneous trip current for the 400A circuit breaker could be as low as 2000A times 0.75 = 1500A (± 25% band). If a fault above 1500A occurs on the loadside of the 90A breaker, both breakers could open. The 90A breaker may unlatch before the 400A breaker.

However, before the 90A breaker can part its contacts and clear the fault current, the 400 amp breaker could have unlatched and started the irreversible contact parting process.

Assume a 4000A short-circuit exists on the loadside of the 90A circuit breaker.

The sequence of events would be as follows:

1. The 90A breaker will unlatch (Point A) and free the breaker mechanism to start the contact parting process.
2. The 400A breaker will unlatch (Point B) and it, too, would begin the contact parting process. Once a breaker unlatches, it will open. At the unlatching point, the process is irreversible. It is similar to pulling a trigger on a gun.
3. At Point C, the 90A breaker will have completely interrupted the fault current.
4. At Point D, the 400A breaker also will have opened, which unnecessarily disrupts power to all other loads.

These two specific circuit breakers with the settings as stated are coordinated for any overcurrent up to approximately 1500A.

However, this is a non-selective system where fault currents are above 1,500A, causing a blackout to all the loads fed by the 400A breaker.

Circuit breaker manufacturers provide Coordination Tables which show circuit breakers of specific types and ampere ratings coordinating to fault values greater than the crossing point where two circuit breaker time-current curves intersect.

The norm in the industry is to display circuit breaker curves for times from 0.01 second to about 100 or 1000 seconds. So typically the circuit breaker curves are not shown with the unlatching curves as in Figure 2.

The following Figure 3 illustrates a 400A (IT = 7×) circuit breaker feeding a 100A circuit breaker.

However, this curve, which is the industry norm, does not show the circuit breaker characteristics below 0.01 second. For coordination analysis, the interpretation of this curve is that these two circuit breakers are coordinated for overcurrents less than approximately 2100A (arrow on Figure 3).

For overcurrents greater than 2100A, these two circuit breakers, with these settings, would not be considered coordinated.

The following is an excerpt from IEEE 1015-2006 “Blue Book” Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems, page 145 5.5.3 Series MCCBs:

Selective coordination is limited to currents below the instantaneous pickup of the lineside circuit breaker. For any fault downstream of the loadside MCCB having a current greater than the instantaneous pickup of the lineside MCCB, both circuit breakers trip, and power is interrupted to unfaulted circuits fed by the lineside circuit breaker.

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2.2 Interpreting Circuit Breaker Curves for Selective Coordination

Figure 4 is the single-line diagram that will be used for the next couple of examples. It has three molded case circuit breakers in series:

1. 1200A main,
2. 400A feeder with
3. the 100A branch circuit.

The other circuit breakers on the one-line diagram supply other circuits and loads. The fault current path from the power source is depicted by the red arrows/lines on the one-line diagram.

For the coordination analysis, faults on both the branch circuit and feeder must be analyzed.

For instance, interpreting the curves for the 100A circuit breaker and the 400A circuit breaker. Their curves intersect in the instantaneous region starting at approximately 3600A.

The 1200A circuit breaker curve intersects the 100A and 400A circuit breaker curves at approximately 6500A.

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2.3 Analysis for branch circuit fault

For a branch circuit fault current less than 3600A on the loadside of the 100A circuit breaker, the 400A and 1200A circuit breakers will be coordinated with the 100A circuit breaker.

If the fault current is greater than 3600A, then the 400A feeder circuit breaker may unnecessarily open and there is a lack of coordination.

If the branch circuit fault is greater than 6500A, then the 1200A main circuit breaker may unnecessarily open, which is a lack of coordination between the 100A, 400A and 1200A circuit breakers.

The reason is, for a fault of greater than 6500A, all three of these circuit breakers are in their instantaneous trip region. Both the 400A and 1200A circuit breakers can unlatch before the 100A circuit breaker clears the fault current.

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2.4 Analysis for feeder circuit fault

For any feeder fault less than 6500 amps on the loadside of the 400A circuit breaker, the 400A and 1200A circuit breakers will be coordinated.

For feeder faults greater than 6500A, the 1200A circuit breaker is not coordinated with the 400A feeder circuit breaker.

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2.5 Conclusion for Figures 4 and 5 coordination analysis

If the maximum available short-circuit current at the 100A branch circuit is less than 3600A and the maximum available short-circuit current at the 400A feeder circuit is less than 6500A, then the circuit path (100A, 400A, and 1200A) is selectively coordinated.

If the maximum available short-circuit current exceeds either of these values, the circuit path is not selectively coordinated.

How does this affect the electrical system?

Look at the single-line diagram in Figure 6.  For any fault current greater than approximately 6500A on the loadside of the 100A circuit breaker, the 1200A and 400A circuit breakers open as well as the 100A circuit breaker.

How does this affect the electrical system?

Look at the single-line diagram in Figure 6. For any fault current greater than approximately 6500A on the loadside of the 100A circuit breaker, the 1200A and 400A circuit breakers open as well as the 100A circuit breaker.

The yellow shading indicates that all three circuit breakers open – branch circuit, feeder and main. In addition, all the loads fed by the other circuit breakers, denoted by the hash shading, are blacked out unnecessarily.

This is due to the lack of coordination between the 100A, 400A and 1200A circuit breakers.

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Source: Selecting Protective Devices by Eaton