Short T.A. Electric Power Distribution Handbook

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Short-Circuit Protection 387

Like a circuit breaker, interruption occurs at a natural current zero. The

interrupting medium of a recloser is most commonly vacuum or oil. The

insulating medium is generally oil, air, a solid dielectric, or SF

. The recloser

control can be electronic, electromechanical (the relay for tripping is electro-

mechanical, and the reclosing control is electronic) or hydraulic. A hydraulic

recloser uses springs and hydraulic systems for timing and actuation.

The interrupting rating of a recloser is based on a symmetrical current

rating. The interrupting current rating does not change with voltage. There

is an exception that some reclosers have a higher interrupting current if

operated at a signiﬁcantly lower voltage than the rating. Smaller reclosers

with a 50- to 200-A continuous rating typically have interrupting ratings

of 2 to 5 kA (these would normally be feeder reclosers). Larger reclosers

that could be used in substations have continuous current ratings as high

as 1120 A and interrupting ratings of 6 to 16 kA. Historically, reclosers with

series coil types had coil ratings of 25, 35, 50, 70, 100, 140, 200, 280, 400,

FIGURE 8.2

Relay curves following the IEEE standardized characteristics for a time dial = 5.

Moderately inverse

Very inverse

Extremely inverse

1.0 10.0 100.02. 20.5. 50.

1.0

10.0

0.2

20.

0.5

Current in multiples of the relay pickup

Time, seconds

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388 Electric Power Distribution Handbook

and 560 A (each rating is approximately 1.4 times higher than the next

lower rating).

Reclosers are tested at a speciﬁed X/R ratio as speciﬁed in ANSI/IEEE

C37.60-1981. A typical test value is X/R = 16. While a lower X/R ratio at the

point of application does not mean you can increase the rating of a recloser,

the recloser must be derated if the X/R ratio is larger than that speciﬁed.

There are some other differences with recloser ratings vs. circuit breaker

ratings (Cooper Power Systems, 1994). Reclosers do not have to be derated

for multiple operations. Reclosers do not have a separate closing and latching

(or ﬁrst-cycle) rating. The symmetrical current rating is sufﬁcient to handle

the asymmetry during the ﬁrst cycle as long as the circuit X/R ratio is lower

than the tested value.

Reclosers have many distribution applications. We ﬁnd reclosers in the

substation as feeder interrupters instead of circuit breakers. An IEEE survey

found that 51% of station feeder interrupting devices were reclosers (IEEE

Working Group on Distribution Protection, 1995). Reclosers are used more

in smaller stations and circuit breakers more in larger stations. Three-phase

reclosers can be used on the main feeder to provide necessary protection

coverage on longer circuits, along with improved reliability. Overhead units

and padmounted units are available. Reclosers are available as single-phase

units, so they can be used on single-phase taps instead of fuses. Another

common application is in autoloop automation schemes to automatically

sectionalize customers after a fault.

Since reclosers are devices built for distribution circuits, some have features

that are targeted to distribution circuit needs. Three-phase units are available

that can operate each phase independently (so a single-phase fault will only

open one phase). Some reclosers have a feature called sequence coordination

to enhance coordination between multiple devices.

The time-current characteristics of hydraulic reclosers have letter designa-

tions: A, B, and C. The A is a fast curve that is used similarly to an instan-

taneous relay element, and the B and C curves have extra delay (“delayed”

and “extra delayed”). For a hydraulically-controlled recloser, the minimum

trip threshold is twice the full-load rating of the trip coil of the recloser and

is normally not adjustable. On electronically-controlled reclosers, the mini-

mum trip threshold is adjustable independently of the rating (analogous to

setting the pickup of a time-overcurrent relay).

8.2.5 Expulsion Fuses

Expulsion fuses are the most common protective device on distribution

circuits. Fuses are low-cost interrupters that are easily replaced (when in

cutouts). Interruption is relatively fast and can occur in a half of a cycle for

large currents. An expulsion fuse is a simple concept: a fusible element made

of tin or silver melts under high current. Expulsion fuses are most often

applied in a fuse cutout. In a fuse tube, after the fuse element melts, an arc

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Short-Circuit Protection 389

remains. The arc, which has considerable energy, causes a rapid pressure

buildup. This forces much of the ionized gas out of the bottom of the cutout

(see Figure 8.3), which helps to prevent the arc from reigniting at a current

zero. The extreme pressure, the stretching of the arc, and the turbulence help

increase the dielectric strength of the air and clear the arc at a current zero.

A fuse tube also has an organic ﬁber liner that melts under the heat of the

arc and emits fresh, non-ionized gases. At high currents, the expulsion action

predominates, while at lower currents, the deionizing gases increase the

dielectric strength the most.

The “expulsion” characteristics should be considered by crews when plac-

ing a cutout on a structure. Avoid placement where a blast of hot; ionized

gas blown out the bottom of the cutout could cause a ﬂashover on another

phase or other energized equipment. Implement and enforce safety proce-

dures whenever a cutout is switched in (because it could be switching into

a fault), including eye protection, arc resistant clothing, and, of course, avoid-

ing the bottom of the cutout.

The speed ratio of a fuse quantiﬁes how steep the fuse curve is. The speed

ratio is deﬁned differently depending on the size of the fuse (IEEE Std.

C37.40-1993):

Speed ratio for fuse ratings of 100 A and under =

Speed ratio for ratings above 100 A =

FIGURE 8.3

Example operation of an expulsion fuse during a fault. (Courtesy of the Long Island Power

Authority.)

melting current at 0.1 seconds

melting current at 300 seconds

melting current at 0.1 seconds

melting current at 600 seconds

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390 Electric Power Distribution Handbook

Industry standards specify two types of expulsion fuses, the most com-

monly used fuses. The “K” link is a relatively fast fuse, and the “T” is

somewhat slower. K links have a speed ratio of 6 to 8. T links have a speed

ratio of 10 to 13. The K link is the most commonly used fuse for transformers

and for line taps. The K and T fuse links are standardized well enough that

they are interchangeable among manufacturers for most applications.

Two time-current curves are published for expulsion fuses: the minimum

melt curve and the maximum total clear curve. The minimum-melt time is

90% of the average melt time to account for manufacturing tolerances. The

total clearing time is the average melting time plus the arcing time plus

manufacturing tolerances. Figure 8.4 shows the two published curves for 50-

A K and T fuse links. The manufacturer’s minimum melt curves for fuses

less than or equal to 100 A normally start at 300 sec, and those over 100 A

start at 600 sec.

The time-current characteristics for K and T links are standardized at three

points (ANSI C37.42-1989). The minimum and maximum allowed melting

current is speciﬁed for durations of 0.1 sec, 10 sec, and either 300 sec (for

fuses rated 100 A or less) or 600 sec (for larger fuses).

Published fuse curves are for no loading and an ambient temperature of

25∞C. Both loading and ambient temperature change the fuse melting char-

acteristic. Load current causes the most dramatic difference, especially when

a fuse is overloaded. Figure 8.5 shows the effect of loading on fuse melting

time. Figure 8.6 through Figure 8.9 show time-current curves for K and T links.

For operation outside of this ambient range, the fuse melting time changes.

The melting characteristic of tin fuse links changes 3.16% for each 10∞C above

or below 25∞C, so a fuse operating in a 50∞C ambient will operate in 92% of

the published time (100% 3.16%). Silver links are less sensitive to tem-

perature (0.9% melting change for each 10∞C above or below 25∞C).

The I

t of a fuse is often needed to coordinate between fuses. Table 8.4

shows the minimum melt I

t of K and T links estimated from the time-current

curves at 0.01 sec. This number is also useful to estimate melting character-

istics for high currents below the published time-current characteristics,

which generally have a minimum time of 0.01 sec.

The 6, 10, 15, 25, 40, 65, 100, 140, and 200-A fuses are standard ratings that

are referred to as preferred fuses. The 8, 12, 20, 30, 50, and 80-A fuses are

intermediate fuses. The designations are provided because two adjacent fuses

(for those below 100 A) will not normally coordinate. A 40 and a 30-A fuse

will not coordinate, but a 40 and a 25-A fuse will coordinate up to some

maximum current. Most utilities pick a standard set of fuses to limit the

number of fuses stocked.

K or T links with tin fuse elements can carry 150% of the nominal current

rating indeﬁnitely. It is slightly confusing that a 100-A fuse can operate

continuously up to 150 A. Overloaded fuses, although they can be safely

overloaded, operate signiﬁcantly faster when overloaded, which could cause

miscoordination. In contrast to tin links, silver links have no continuous

overload capability.

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Short-Circuit Protection 391

FIGURE 8.4

Minimum melt and total clearing curves for a T and K link (50 A).

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

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392 Electric Power Distribution Handbook

Other nonstandard fuses are available from manufacturers for special pur-

poses. One type of specialty fuse is a fuse even slower than a T link that is

used to provide better coordination with upstream circuit breakers or reclos-

ers in a fuse saving scheme. Another notable type of specialty fuse is a surge-

resistant fuse that responds slowly to fast currents (such as surges) but faster

to lower currents. These achieve better protection on transformers for sec-

ondary faults and faster operation for internal transformer failures while at

the same time reducing nuisance fuse operations due to lightning.

Expulsion fuses under oil are another fuse variation. These “weak-links”

are used on CSP (completely self-protected) transformers and some pad-

mounted and vault transformers. Since they are not easily replaced, they

have very high ratings — at least 2.5 times the transformer full load current

and much higher if a secondary circuit breaker is used.

Transformers on underground circuits use a variety of fuses. For pad-

mounted transformers, a common fuse is the replaceable Bay-O-Net style

expulsion fuse. The time-current characteristics of this fuse do not follow

one of the industry standard designations.

8.2.5.1 Fuse Cutouts

The cutout is an important part of the fuse interrupter. The cutout determines

the maximum interrupting capability, the continuous current capability, the

load-break capability, the basic lightning impulse insulation level (BIL), and

the maximum voltage. Cutouts are typically available in 100, 200, and 300-

A continuous ratings [ANSI standard sizes (ANSI C37.42-1989)].

FIGURE 8.5

Effect of loading on fuse melting time. (Adapted from Cooper Power Systems, Electrical Distri-

bution — System Protection, 3rd ed., 1990. With permission from Cooper Industries, Inc.)

tin

silver

0 20 40 60 80 100

100

150

200

Melting time in percent of published curves

Preload current in percent of link rating

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Short-Circuit Protection 393

FIGURE 8.6

Minimum melt curves for K links. (S&C Electric Company silver links.)

100

140 200

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

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394 Electric Power Distribution Handbook

FIGURE 8.7

Maximum total clear curves for K links. (S&C Electric Company silver links.)

100

140 200

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

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Short-Circuit Protection 395

FIGURE 8.8

Minimum melt curves for T links. (S&C Electric Company silver links.)

100

140 200

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

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396 Electric Power Distribution Handbook

FIGURE 8.9

Maximum total clear curves for T links. (S&C Electric Company silver links.)

100

140 200

0.01

0.1

1.0

10.0

100.0

Current, amps

Time, seconds

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