700.16 Emergency Illumination
What does this section require?
Emergency lighting systems cannot allow a blackout in any area
requiring emergency illumination due to the failure of any one element of the lighting system. Such failures could be caused by the
burning out of a light bulb or the opening of a branch circuit protective device due to a faulted ballast. The solution to the burnt out
light bulb is to have additional bulb(s) in the area. The solution to
the open branch circuit protective device is to install listed supplementary fuses on each ballast. In that way, a faulted ballast would
be taken off the line by the supplementary fuse, not by the branch
circuit protective device, allowing the rest of the emergency system
to remain energized.
VIOLATION
Fixture No. 1
20A
Fixture No. 2
Branch
(Opens)
Fixture No. 3
COMPLIANCE
Fixture No. 1
Fixture No. 2
20A
Branch
(Remains
energized)
Fixture No. 3
(Open)
Fault
Fixture No. 4
The fault in Fixture #3 will open just the supplementary fuse. The 20
ampere branch circuit device does not open and Fixtures 1, 2 and 4
remain energized, preventing a blackout.
Fault
Fixture No. 4
The fault in Fixture #3 causes the 20 ampere branch circuit overcurrent
device to open, causing a blackout in the entire area.
700.25 Emergency System Overcurrent Protection Requirements (FPN)
What is the meaning of this fine print note?
In order to maximize the reliability of emergency systems, the overcurrent devices must be selectively coordinated. Time-current
curves of both fuses and circuit breakers must be examined to
determine whether or not only the overcurrent device closest to a
fault opens. If additional upstream devices open, the system is not
selectively coordinated, causing additional sections of the emergency system to black out and therefore, reducing the reliability of
that system.*
BLACKOUT PREVENTION!
Increased Reliability
COMPLIANCE
1000A
Not
Open
BLACKOUT!
VIOLATION
1000A
I.T.=10x
Opens instantaneous at 10,000 A
225A
Not
Open
225A
I.T.=8x.
Opens instantaneous at 1,800 A
20A
I.T.=8A
Opens
22,000 Amp
Short-Circuit
Reduced Reliability
Fault exceeding the instantaneous trip setting of all three circuit breakers in series will open all three. This will blackout the entire emergency
system.
*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.
20A
Opens
22,000 Amp
Short-Circuit
Fault opens the nearest upstream fuse, allowing other circuits to remain
energized. Reliability of the emergency system is increased.