240.12 System Coordination or Selectivity
1. The 90 ampere breaker will unlatch (Point A) and free the
breaker mechanism to start the actual opening process.
2. The 400 ampere breaker will unlatch (Point B) and it, too, would
begin the opening process. Once a breaker unlatches, it will open.
The process at the unlatching point is irreversible.
3. At Point C, the contacts of the 90 ampere breaker finally open
and interrupt the fault current.
4. At Point D, the contacts of the 400 ampere breaker open. . .the
entire feeder is “blacked out”!
What is the importance of this section?
Whenever a partial or total building blackout could cause
hazard(s) to personnel or equipment, the fuses and/or circuit
breakers must be coordinated in the short-circuit range. It is
acceptable for a monitoring system to be used to indicate an
overload condition, if the overcurrent protective devices cannot be
coordinated in the overload region. However, in the vast majority
of cases, both circuit breakers and fuses will be able to be
coordinated in the overload range, so the monitoring systems will
seldom be required. Typical installations where selective
coordination would be required include elevator circuits, hospitals,
industrial plants, office buildings, schools, government buildings,
military installations, high-rise buildings, or any installation where
continuity of service is essential.*
Example of Non-Selective System.
1,000
800
600
400
300
* Footnote: See also Section 4-5.1 of NFPA 110 (Emergency and Standby Power
Systems) and Sections 3-3.2.1.2(4) & 3-4.1.1.1 of NFPA 99 (Health Care Facilities) for
additional information on selective coordination.
200
100
80
60
40
30
VIOLATION
225A
I.T.=8x
20A
I.T.=8x
Opens
20
Opens
Opens
22,000 Amp
Short-Circuit
Fault exceeding the instantaneous trip setting of all 3 circuit breakers in series will
open all 3. This will blackout the entire system.
4
3
90A
2
1
.8
.6
.4
.3
ShortCircuit
.2
.1
.08
.06
.04
.03
COMPLIANCE
1000A
400A
400 AMP
Circuit Breaker
I.T. = 5X
90 AMP
Circuit
10 Breaker
8
6
TIME IN SECONDS
1000A
I.T.=10x
POINT D
.02
Not
Open
POINT C
.01
.008
.006
POINT B
.004
.003
POINT A
30,000
30,000
20,000
6,000
8,000
10,000
3,000
4,000
2,000
600
800
1,000
.001
300
400
Not
Open
100
225A
200
.002
CURRENT IN AMPERES
20A
Now, let’s take the case of fuse coordination. When selective
coordination of current-limiting fuses is desired, the Selectivity
Ratio Guide (next page) provides the sizing information necessary.
In other words, it is not necessary to draw and compare curves.
Current-limiting fuses can be selectively coordinated by
maintaining at least a minimum ampere rating ratio between the
main fuse and feeder fuses and between the feeder fuse and
branch circuit fuses.
These ratios are based on the fact that the smaller downstream
fuses will clear the overcurrent before the larger upstream fuses
melt. An example of ratios of fuse ampere ratings which provide
selective coordination is shown in the one-line circuit diagram.
Opens
22,000 Amp
Short-Circuit
19-B
Fault opens the nearest upstream fuse, localizing the fault to the equipment
affected. Service to the rest of the system remains energized.
If the ampere rating of a feeder overcurrent device is larger than the
rating of the branch circuit device, are the two selectively coordinated?
No. A difference in rating does not in itself assure coordination. For
example, a feeder circuit breaker may have a rating of 400
amperes and the branch breaker 90 amperes. Under overload
conditions in the branch circuit, the 90 ampere breaker will open
before, and without, the 400 ampere breaker opening. However,
under short-circuit conditions, not only will the 90 ampere device
open, the 400 ampere may also open. In order to determine
whether the two devices will coordinate, it is necessary to plot their
time-current curves as shown. For a short-circuit of 4000 amperes:
2:1 (or more)
LPS-RK90SP
LPS-RK400SP
Short-Circuit
24
240.12 System Coordination or Selectivity
*Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout Prevention.
Circuit
Current Rating
Type
Trade Name &
Class
Line-Side Fuse
601 to
6000A
601 to
4000A
0
to
600A
601 to
6000A
0 to
600A
0 to
1200A
0 to
600A
0 to
60A
Buss
Symbol
Time- LOW-PEAK® KRP-C–SP
Delay YELLOW™ (L)
Time- LIMITRON® KLU
Delay (L)
LOW-PEAK® LPN-RK–SP
YELLOW™ LPS-RK–SP
Dual (RK1)
Ele(J)
LPJ–SP**
ment FUSETRON® FRN-R
(RK5)
FRS-R
LIMITRON® KTU
(L)
Fast- LIMITRON® KTN-R
Acting (RK1)
KTS-R
T-TRON®
JJN
(T)
JJS
LIMITRON® JKS
(J)
Time- SC
SC
Delay (G)
Load-Side Fuse
601-6000A
601-4000A
TimeTimeDelay
Delay
LOW-PEAK® LIMITRON®
YELLOW™
(L )
(L )
0-600A
Dual-Element
Time-Delay
LOW-PEAK®
YELLOW™
(RK1)
601-6000A
Fast-Acting
0-1200A
FUSETRON® LIMITRON®
LIMITRON®
T-TRON®
0-60A
TimeDelay
LIMITRON® SC
(T)
JJN
JJS
2:1
(J)
JKS
(G)
SC
2:1
(RK1)
KTN-R
KTS-R
2:1
2:1
N/A
2.5:1
2:1
(RK5)
FRN-R
FRS-R
4:1
2:1
2:1
2:1
2:1
4:1
2:1
2:1
2:1
2:1
N/A
–
–
2:1
2:1
8:1
–
3:1
3:1
3:1
4:1
–
–
–
–
2:1
1.5:1
2:1
1.5:1
8:1
2:1
–
–
3:1
1.5:1
3:1
1.5:1
3:1
1.5:1
4:1
1.5:1
2:1
2.5:1
2:1
2:1
6:1
2:1
2:1
2:1
2:1
N/A
–
–
3:1
3:1
8:1
–
3:1
3:1
3:1
4:1
–
–
3:1
3:1
8:1
–
3:1
3:1
3:1
4:1
–
–
2:1
2:1
8:1
–
3:1
3:1
3:1
4:1
–
–
3:1
3:1
4:1
–
2:1
2:1
2:1
2:1
KLU
2:1
(L )
KTU
0-600A
LPN-RK–SP
LPS-RK–SP
2:1
KRP-C–SP
(J)**
LPJ–SP
0-600A
Fast-Acting
lowered to permit closer fuse sizing. Plot fuse curves or consult with Bussmann.
* Note: At some values of fault current, specified ratios may be
®
General Notes: Ratios given in this table apply only to Buss fuses. When fuses are within the same case size, consult Bussmann.
** Consult Bussmann for latest LPJ—SP ratios.
.
240.13 Ground Fault Protection of Equipment on Buildings or Remote Structures
What does this section require?
Equipment ground fault protection of the type required in 230.95 is
required for each disconnect rated 1000 amperes or more in
480Y/277V solidly grounded systems that will serve as a main
disconnect for a separate building or structure. Refer to 215.10
and 230.95.
High Voltage
Service
G.F.P. Not
Required
Building A
800A
480Y/277V
G.F.P. Not
Required
Building B
Note: G.F.P. that is not current-limiting may not protect system
components. See 110.10 and 250.1 (FPN).
1000A or Greater
480Y/277V
G.F.P.
Required
240.21 Location Requirements for Overcurrent Devices and Tap Conductors
made to a switchboard bus for an adjacent panel, such as an
emergency panel, the use of cable limiters is recommended as
supplementary short-circuit protection of the tapped conductor.
These current-limiting cable limiters are available in sizes designed
for short-circuit protection of conductors from 12 AWG to 1000
kcmil. They provide current-limiting short-circuit protection but not
overload protection.
240.21 Location in Circuit
Requires overcurrent protection to be provided in each
ungrounded circuit conductor and be located at the point where
the conductors receive their supply except as specified in
240.21(A)-(G). No conductor supplied per 240.21(A)-(G) shall
supply another conductor, except through an overcurrent device
meeting the requirements of 240.4. In other words, “you can’t tap
a tap!”
The most common use of tap conductors for feeders, 240(B), and
transformer secondary conductors, 240(C), are the 10 foot, 25 foot
and outside tap conductor rules. It is important to realize that
although they allow for unprotected lengths of conductors, in
almost all cases, termination in a single or group of overcurrent
protection devices is required. In addition, it may be necessary to
meet other requirements for panelboards, 408.16, and
transformers, 450.3.
Note: Smaller conductors tapped to larger conductors can be a
serious hazard. If not adequately protected against short-circuit
conditions (as required in 110.10 and 240.1), these
underprotected conductors can vaporize or incur severe
insulation damage. Molten metal and ionized gas created by a
vaporized conductor can envelop other conductors (such as bare
bus), causing equipment burn-down. Adequate short-circuit
protection is recommended for all conductors. When a tap is
25