TRI-SERVICE ELECTRICAL WORKING GROUP (TSEWG)
07/16/08
TSEWG TP-10: ARC FLASH LEVELS FOR FUSED VOLTAGE INPUTS TO
ELECTRICITY METERS
INTRODUCTION.
NAVFAC Specification 26 27 13.00 20, Electricity Metering, requires fusing for the
voltage input to an electricity meter and does not recommend the use of potential
transformers for 480-volt applications. This Technical Paper discusses the typical arc
flash level downstream of a fused input to a typical electricity meter.
For the purposes of analysis, two cases are considered:
•
A relatively large distribution transformer with a large available short circuit current
on the secondary side.
•
A relatively small distribution transformer with a small available short circuit current
on the secondary side.
The above two cases likely bound typical installations.
ANALYSIS CONFIGURATION.
A simple EasyPower model was developed with two distribution transformers as shown
in Figure 1. Transformer TX-1 is a 2000 kVA transformer and Transformer TX-2 is a 45
kVA transformer. The output of each transformer has 10 feet of #10 awg conductor
representing the supply to the electricity meter. Each conductor is protected by a Class
RK-5 600-volt fuse, which is a typical fuse type for this application.
Figure 1 EasyPower Model
The available short circuit current (kA) for this configuration is shown in Figure 2. As
shown, the large transformer has a short circuit current of about 36 kA and the small
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TRI-SERVICE ELECTRICAL WORKING GROUP (TSEWG)
07/16/08
transformer has a short circuit current of about 1.4 kA. Conductor resistance reduces
the short circuit current available at the meter.
Figure 2 Short Circuit Current
ARC FLASH RESULTS.
Figure 3 shows a typical arc flash result for this configuration.
Figure 3 Arc Flash Level
Figure 3 shows a Category 0 arc flash level; this particular result is based on 20 ampere
fuses in each line. These results remain at Category 0 for fuse sizes up to 110
amperes. The results are based on the time-current response of the fuse. The lower
short circuit current level of the smaller transformer eventually becomes limiting
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TRI-SERVICE ELECTRICAL WORKING GROUP (TSEWG)
07/16/08
because of the longer fuse response duration for the lower short circuit current. Table 1
summarizes the results in terms of cal/cm2 at a working distance of 18 inches.
Table 1 Arc Flash Levels as a Function of Fuse Size (cal/cm2)
Transformer
T-1
(2000 kVA)
T-2
(45 kVA)
10
amp
20
amp
50
amp
100
amp
110
amp
125
amp
150
amp
200
amp
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.4
0.0
0.0
0.1
1.1
2.0
4.2
10.4
15.9
Figure 4 shows a coordination plot for the 200 ampere fuses protecting the meter on the
45 kVA transformer. The transformer secondary-side short circuit current of about 1.3
kA will take several seconds to clear.
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TRI-SERVICE ELECTRICAL WORKING GROUP (TSEWG)
07/16/08
Figure 4 Coordination Plot
1000
4 5 6 7 8 9 10
2
CURRENT IN AMPERES AT 480 VOLTS
3 4 5 6 7 8 9 100
2 3 4 5 6 7 8 9 1000 2 3 4 5 6 7 8 9 10000 2
3 4 5 67
1000
700
700
500
400
300
500
400
300
100
TIME IN SECONDS
200
FS-3
BUSS
(Std)
FRN-R(RK-5)
200A
100
70
70
50
40
30
50
40
30
20
20
10
10
7
7
5
4
3
5
4
3
2
2
1
1
.7
.7
.5
.4
.3
.5
.4
.3
.2
.2
.1
.1
.07
.07
.05
.04
.03
.05
.04
.03
.02
.01
.02
FS-3
1425A
4 5 6 7 8 9 10
2
3 4 5 6 7 8 9 100
2
3 4 5 6 7 8 9 1000
2
TIME IN SECONDS
200
3 4 5 6 7 8 9 10000 2
3 4 5 67
.01
CURRENT IN AMPERES AT 480 VOLTS
In summary, fusing the voltage input to the electricity meter provides adequate arc flash
protection. Although larger fuse sizes were evaluated here, the fuses used should
typically be 20 amperes or less to provide conductor protection. Potential transformers
should not be necessary for 480-volt applications.
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