Installation
19
AlphaEclipse 3600 Series A Sign Installation Instructions (9711-8001C)
September 28, 2004
Ground fault protection
Sign with G
r
ound Fault P
r
otection
= g
r
o
un
d fa
u
lt c
urr
e
n
t path
Se
r
vice
Light
n
i
n
g
elect
r
ode
Light
n
i
n
g
elect
r
ode
Panel
Sign
N
N
G
r
ound fault
Beca
us
e a low
r
e
s
i
s
ta
n
ce pathway ha
s
bee
n
p
r
ovided,
the la
r
ge amo
un
t of c
urr
e
n
t that flow
s
alo
n
g thi
s
pathway
will ca
us
e a b
r
eake
r
to t
r
ip a
n
d the ci
r
c
u
it to ope
n
.
Sign without G
r
ound Fault P
r
otection
= g
r
o
un
d fa
u
lt c
urr
e
n
t path
Se
r
vice
Light
n
i
n
g
elect
r
ode
Light
n
i
n
g
elect
r
ode
Panel
Sign
N
N
G
r
ound fault
No g
r
o
un
d
co
nn
ectio
n
No path fo
r
fa
u
lt c
urr
e
n
t
Beca
us
e of the
r
elatively high
r
e
s
i
s
ta
n
ce of a
n
ea
r
th g
r
o
un
d,
o
n
ly a
s
mall amo
un
t of c
urr
e
n
t will flow th
r
o
u
gh the light
n
i
n
g elect
r
ode.
Thi
s s
mall c
urr
e
n
t flow will p
r
obably
n
ot be e
n
o
u
gh to ope
n
the ci
r
c
u
it.
A
s
a
r
e
su
lt, the
s
ig
n
ca
s
e will become a
s
hock haza
r
d
a
n
d po
ss
ibly a fi
r
e haza
r
d if the c
urr
e
n
t level i
s
high e
n
o
u
gh.
What is a ground fault?
It’s when a “hot” wire
unintentionally
makes contact with metal, like an electrical outlet box or the case of a sign. If a sign is wired
properly, a circuit breaker will trip (or “open”) because too much electrical current is flowing. The ground fault will have to be corrected
before the circuit breaker can be closed.
How do you protect against ground faults?
Provide a ground fault current path with so
little
resistance (basically just the resistance of the power wires) that a
huge
amount of
current tries to flow. For example, imagine a sign is powered by 120V and is connected to a 20 ampere circuit. If this sign tries to draw
more than 20 amps of current, the circuit breaker will trip and the circuit will open. Let’s say that this sign is 300 feet away from its power
source and that the total resistance of this wire is 0.4 ohms. Using Ohm’s Law, the fault current = 120V divided by 0.4 ohms = 300 amps!
This amount of current will cause the circuit breaker to open very quickly — removing the shock threat.
Why can’t lightning rods be used to protect against ground faults?
A lightning rod (or earth ground) may have too
much
resistance and so not enough current will flow through it to cause a circuit
breaker to open. For example, imagine an earth ground has a resistance of 10 ohms, which is low. Using Ohm’s Law again, the fault
current = 120V divided by 10 ohms = 12 amps. This is not enough current to cause the 20 amp circuit breaker to open. This means the
ground fault would not be cleared and dangerous levels of current would be present on the sign’s case
and
near the ground rod itself.
A “ground fault” protected circuit is
different from a “ground fault interrupt”
protected circuit.
A sign
should
be ground fault protected
as shown.
However, a sign
should not
be
connected to a ground fault interrupt
(GFI) protected circuit.
Adaptive Explains
