Whis is 4.889 typically used for LV breaker X/R ratio

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philly

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Does anyone know why the number 4.889 is typically used for the test X/R rations of most LV breakers. I have seen this number on alot of LV breakers listed as the tested X/R ratio, and I am looking for references.
 
mivey

mivey

Senior Member
I believe it comes from UL 489. Here is a list I got from somewhere:

Panelboards < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 67

10 kA < Panelboards < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 67

Panelboards > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 67

Motor Control Centers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 845

10 kA < Motor Control Centers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 845

Motor Control Centers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 845

Switchboards < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 891

10 kA < Switchboards < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 891

Switchboards > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 891

Transfer Switches < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 1008

10 kA < Transfer Switches < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 1008

Transfer Switches > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 1008

Switchgear : pf= 0.15, X/R= 6.591, S2P= 2.292, Std: ANSI C37.50

Molded Case Circuit Breakers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 489

10 kA < Molded Case Circuit Breakers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 489

Molded Case Circuit Breakers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 489

Insulated Case Circuit Breakers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 489

10 kA < Insulated Case Circuit Breakers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 489

Insulated Case Circuit Breakers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 489

Power Circuit Breakers : pf= 0.15, X/R= 6.591, S2P= 2.292, Std: UL 1066

Power Circuit Breakers (Fused) : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 1066

Fuses < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 248-1

Fuses > 10 kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 248-1

HV/MV Switchgear (kA Rating Basis) : pf= 0.0587, X/R= 17, S2P= 2.59, Std: ANSI C37.09-1999

HV/MV Switchgear (MVA Rating Basis) : pf= 0.0665, X/R= 15, S2P= 2.561, Std: ANSI C37.010-1979

MV E2 Motor Starter : pf= 0.0665, X/R= 15, S2P= 2.561, Std: UL 347

HV/MV Power Fuses : pf= 0.0665, X/R= 15, S2P= 2.561, Std: ANSI C37.41
 
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philly

Senior Member
mivey said:
I believe it comes from UL 489. Here is a list I got from somewhere:

Panelboards < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 67

10 kA < Panelboards < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 67

Panelboards > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 67

Motor Control Centers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 845

10 kA < Motor Control Centers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 845

Motor Control Centers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 845

Switchboards < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 891

10 kA < Switchboards < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 891

Switchboards > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 891

Transfer Switches < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 1008

10 kA < Transfer Switches < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 1008

Transfer Switches > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 1008

Switchgear : pf= 0.15, X/R= 6.591, S2P= 2.292, Std: ANSI C37.50

Molded Case Circuit Breakers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 489

10 kA < Molded Case Circuit Breakers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 489

Molded Case Circuit Breakers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 489

Insulated Case Circuit Breakers < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 489

10 kA < Insulated Case Circuit Breakers < 20 kA : pf= 0.3, X/R= 3.18, S2P= 1.941, Std: UL 489

Insulated Case Circuit Breakers > 20kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 489

Power Circuit Breakers : pf= 0.15, X/R= 6.591, S2P= 2.292, Std: UL 1066

Power Circuit Breakers (Fused) : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 1066

Fuses < 10 kA : pf= 0.5, X/R= 1.732, S2P= 1.645, Std: UL 248-1

Fuses > 10 kA : pf= 0.2, X/R= 4.899, S2P= 2.159, Std: UL 248-1

HV/MV Switchgear (kA Rating Basis) : pf= 0.0587, X/R= 17, S2P= 2.59, Std: ANSI C37.09-1999

HV/MV Switchgear (MVA Rating Basis) : pf= 0.0665, X/R= 15, S2P= 2.561, Std: ANSI C37.010-1979

MV E2 Motor Starter : pf= 0.0665, X/R= 15, S2P= 2.561, Std: UL 347

HV/MV Power Fuses : pf= 0.0665, X/R= 15, S2P= 2.561, Std: ANSI C37.41
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Thanks, this list will provide a good reference and is pretty much in line with the values I've seen for the various pieces of equipemnt.

I'm just curious why they choose the power factors and X/R ratios listed above for the testing rating.
 
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jim dungar

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G._S._Ohm said:
I'd think a fuse would be almost all resistive. Are these lower limits on PF?
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These have nothing to due with the construction of the fuse.
These values are how the test sources are configured to produce the correct amount of fault currents. A proper device short circuit evaluation, will make rating adjustments depending on the calculated/actual circuit parameters versus the particular device's standard test values. This difference is one reason that some engineers want a 10-20% 'safety margin' between the available short circuit current value and a device's AIC/SCCR.
 
mivey

mivey

Senior Member
philly said:
I'm just curious why they choose the power factors and X/R ratios listed above for the testing rating.
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I'm not sure of the specifics but you will notice the conditions get worse for the higher rated breakers since it is assumed they might be exposed to higher asymmetric conditions. The tested pf ranges are supposed to cover most conditions.

However, that is not always the case and should be looked at if you are close to the device maximum rating (IEEE C37.13 says within 80% of a circuit breaker's short-circuit current rating). Then you would need to use a multiplier to determine the required rating of the device (either from a table or calculated).

From IEEE C37.13:
10.1.4.3 Power Factor Considerations
Normally the short-circuit power factor (X/R) of a system need not be considered in applying circuit breakers. This is based on the fact that the power factors on which the ratings of the circuit breakers in this standard have been established amply cover most applications. For unfused circuit breakers this power factor is 15% (X/R ratio of 6.6). For fused circuit breakers the power factor is 20% (X/R ratio of 4.9), which is consistent with the standards established for fuses. The high short-circuit current rating of fused circuit breakers makes the need to consider power factor even more unlikely. There are, however, some specific applications when the available short-circuit current approaches 80% of the circuit breaker short-circuit current rating, which may require additional consideration because of lower shortcircuit power factors. These considerations are as follows:

? Local generation at circuit-breaker voltage in unit sizes greater than 500 kVA
? Gas-filled and dry-type transformers in sizes 1000 kVA and above; all types 2500 kVA and above
? Network systems
? Transformers with impedances higher than those specified in the ANSI C57 series of standards
? Current-limiting reactors at circuit-breaker voltage in source circuits
? Current-limiting busway at circuit-breaker voltage in source circuits
Click to expand...
 
mivey

mivey

Senior Member
Also from the IEEE Blue Book:

3.40 Circuit-breaker evaluation for X/R ratio or short-circuit power factor

LVPCBs in general are evaluated for short-circuit interrupting capability on a first-halfcycle basis. As indicated, MCCBs can sometimes operate so quickly that they function in a current-limiting mode, which means they operate to limit short-circuit current before the first current peak is reached. As the peak current is a function of the offset of the rms symmetrical current wave, which is in turn a function of the power factor or the X/R ratio of the circuit, the fact that (1) LVPCBs are tested with an X/R ratio of 6.6 and (2) MCCBs and ICCBs are tested with X/R ratios of 6.6 to 4.89, 3.8 to 3.18, and 1.98 to 1.75, depending on interrupting rating, means they have to be evaluated differently. See Table 3-18 for a listing of the power factor ranges from which the MCCB and ICCB X/R ratio ranges are derived.
Click to expand...
 
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