4.0 PROTECTIVE DEVICE APPLICATION
The criteria / guidelines for protective device application on distribution circuits are listed as
follows:
1) The protective device voltage rating must meet or exceed the system operating voltage.
2) The protective device interrupting capability must exceed the maximum bolted fault
current (3-phase or phase-ground) at the point of application.
3) The protective device must be able to detect minimum end-of-line bolted faults within its
zone of protection with a margin of 1.5 or greater. A margin of 2.0 or greater is
preferred.
4) The protective device must coordinate with upstream and downstream protective devices.
Lock out coordination must be achieved when two protective devices are connected in
series however; trip coordination may or may not be achieved.
5) The protective device must be able to carry maximum continuous loads without damage
or false tripping. Protective device application is for fault isolation, not overload
protection.
6) The protective device must be able to carry load during contingency conditions without
damage or false tripping. Contingency conditions include cold-load pickup and circuit
switching.
Examples of protective device application on various types of distribution circuits are provided
in Section 8.0.
4.1 Fuse Application
Fuse voltage rating should meet or exceed the system operating voltage.
Fuse symmetrical interrupting rating should exceed the maximum bolted fault level at the point
of application. In areas near 4kV substations, the interrupting rating of T-link fuses may be
exceeded. In these situations, SMU-20, SM-4, or SM-5 fuses need to be installed to meet
required interrupting capability. In areas near 12kV substations, the interrupting rating of T-link
fuses may be exceeded. In these situations, SMU-20 fuses need to be installed to meet required
interrupting capability.
Fuse total clear curve should detect minimum end-of-line bolted faults with a margin of 1.5 or
greater. A fault detection margin of 2.0 or greater is preferred.
Maximum normal and contingency loads should not exceed the fuse manufacturer’s published
continuous and 8 hour ratings respectively.

Coordination requirements are as follows:
Fuse-Fuse Coordination
The upstream fuse must not be damaged by a fault beyond the downstream fuse. The total
clearing time of the downstream fuse should be less than 75% of the upstream fuse minimum
melt time. Fuse-fuse coordination is shown in Tables 4-1 through 4-4. In general, for fuses of
the same curve shape, fuses in series should be offset by two sizes in order to coordinate.
Hydraulic Recloser-Fuse Coordination
The upstream recloser must not lock out for a fault beyond the downstream fuse. To ultimately
achieve lock out coordination using hydraulic reclosers, a time margin of at least 0.1 second
between the total clearing time of the fuse and the recloser delayed time is required. Also, the
recloser can be used to clear temporary faults downstream of the fuse, without damaging the
fuse, by operation on its fast curve. To achieve fuse savings, a time margin of 0.1 second
between the recloser fast curve and the fuse minimum melt curve is required to prevent
damaging the fuse. As such, recloser-fuse combinations that meet these two constraints over the
applicable fault current range are preferred. However, recloser-fuse combinations that provide
lock out coordination are required. Recloser-fuse coordination is shown in Table 4-5.
Circuit Breaker/Electronically Controlled Recloser-Fuse Coordination
The upstream circuit breaker/electronically controlled recloser must not lock out for a fault
downstream of the fuse. To ultimately achieve lock out coordination, a time margin of at least
0.1 second between the total clearing time of the fuse and the overcurrent relay time is required.
The circuit breaker/electronically controlled recloser may be used to clear temporary faults
downstream of the fuse, without damaging the fuse, by operation of an instantaneous relay. To
achieve fuse savings, a time margin of 0.1 second between the instantaneous relay operating time
plus the breaker clearing time and the fuse minimum melt curve is required to prevent damaging
the fuse. As such, circuit breaker/electronically controlled recloser-fuse combinations that meet
these two constraints over the applicable fault current range are preferred. However, circuit
breaker/electronically controlled recloser-fuse combinations that provide lock out coordination
are required.
Circuit breaker-fuse coordination is discussed further in Section 6.0. The largest fuses that
coordinate with the standard breaker settings are listed in Section 6.2. Standard electronically
controlled recloser-fuse coordination is shown in Table 4-5.Line Recloser Application

4.2 Line Recloser Application
Recloser voltage rating should meet or exceed the system operating voltage.
Recloser symmetrical interrupting rating should exceed the maximum bolted fault level at the
point of application.
Recloser MTO must be less than the minimum end-of-line bolted faults with a margin of 1.5 or
greater. A margin of 2.0 or greater is preferred. Hydraulic recloser MTO is typically twice its
rating. Hydraulic reclosers with an X designation (400X or 560X) have an MTO of 1.4 times its
rating. Relay / Microprocessor controlled recloser MTO is based on the trip settings of the phase
and ground elements.
Maximum normal and contingency loads should not exceed the recloser manufacturer’s
published continuous and 8 hour ratings respectively. Per Cooper Power Systems, hydraulic
recloser emergency ratings are 125% of the trip coil rating.
Typically, hydraulic reclosers will have two fast and two delayed operations before locking out.
S&C Intellirupters are electronically controlled reclosers used for backbone protection. See
section 5.2 for more details.
S&C Tripsavers are small reclosers that fit universal cutout fuse mountings. Tripsavers are used
in place of tap fuses for fuse saving. See Section 5.2 for more details.
Coordination requirements are as follows:
Recloser-Fuse Coordination
See section 4.1.
Recloser-Sectionalizer Coordination
See section 4.3.
Recloser-Recloser Coordination
The upstream recloser must not lock out for a fault beyond the downstream recloser. To achieve
lock out coordination, a time margin of at least 0.2 second between the delayed curves of both
reclosers is required. Trip coordination may be achieved between electronically controlled
reclosers and hydraulic reclosers however; trip coordination will rarely be achieved between
hydraulic reclosers. The upstream reclosers may trip on their fast curves when the downstream
reclosers trip on their fast curves. If not, then the upstream reclosers may trip on their fast curves
when the downstream reclosers trip on their delayed curves. Recloser-recloser coordination is
shown in Table 4-6.

Circuit Breaker-Recloser Coordination
The upstream circuit breaker must not lock out for a fault beyond the downstream recloser. To
achieve lock out coordination, a time margin of at least 0.3 second is required between recloser
delayed operation and the overcurrent relay curve for microprocessor based relays with
instantaneous reset.
For circuit breakers with electromechanical relays which have time delay reset, a time margin of
at least 0.1 second is required between the total time of all delayed operations of the hydraulic
recloser and the overcurrent relay curve of the circuit breaker. Also, a time margin of at least 0.3
second is required between the delayed operations the electronically controlled recloser and the
overcurrent relay curve of the circuit breaker. To achieve trip coordination, the instantaneous
relay setting of the circuit breaker must be at least 120% of the maximum bolted fault current at
the recloser. However, in order to provide fuse savings on the remainder of the circuit, the
instantaneous relay setting is not usually altered to achieve trip coordination.
Circuit breaker-recloser coordination is discussed in Section 6.0. The largest reclosers that
coordinate with the standard breaker settings are listed in Section 6.2.

4.4 Distribution Transformer Protection
Each phase of a transformer bank is protected by a fuse.
Fuse voltage rating should meet or exceed the system operating voltage.
Fuse symmetrical interrupting rating should exceed the maximum bolted fault level at the point
of application.
Fuse total clear curve should detect transformer low-side bolted faults with a margin 1.5 or
greater and protect the transformer damage curve. For single-phase and three-phase wye-wye
transformers, low-side faults are reflected to the high-side via the turns ratio. For three-phase
delta-wye transformers, low-side phase-to-ground faults are reflected to the high-side via the
turns ratio x 0.577. Low-side multi-phase faults are reflected to the high-side as the three-phase
fault level via the turns ratio.
Fuse total clear curve should plot below the transformer damage curve. Damage curves are
developed per ANSI C57.12 standard.
For a category I transformer (5-500kVA single or three-phase) the damage curve is 25 times full
load current at 2.0 seconds and 12.5 times full load current at 8.0 seconds (I2T = 1250).
For a category II transformer (501-1667kVA single-phase and 501-5000kVA three-phase) the
damage curve is 2.0 seconds at maximum low-side fault, and 4.0 seconds at 0.7 x maximum lowside fault. For delta-wye transformers the damage curve must be shifted by 0.577 for phaseground faults. Maximum low-side faults are calculated assuming infinite source bus. Note the
category II damage curve includes a frequent fault branch based on transformer impedance
which must be protected.
Fuse minimum melt curve should exceed transformer magnetizing inrush current and allow a
minimum 0.3 seconds coordination time with low-side protection. Conservative estimates of the
worst case magnetizing inrush currents are 12 times the nameplate current rating for 0.1 second
and 25 times the nameplate current rating for 0.01 second.
Maximum normal and contingency loads should not exceed the fuse manufacturer’s published
continuous and 8 hour ratings respectively.
Distribution transformer loading and impedance data is shown in Table 4-7. This data was used
as the basis to determine the appropriate fuse size.

Table 5-1 Standard Fuse Mounting and Link Ratings

* For use in 100 Amp cutout cartridge only
** For use in 200 Amp cutout cartridge only
+ 200T fuses will not coordinate with standard settings, not to be used for new construction

* Previously installed equipment—not for new construction

Table 5-5 Standard Recloser Ratings (4kV and 12kV)

* See standard settings, Table 5-7
**Rating of device ampacity; minimum trip is determined by controller setting