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AlphaEclipse 3600 Series A Sign Installation Instructions (9711-8001C)
Ground fault protection
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
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.
Installation
Sign with Ground Fault Protection
= ground fault current path
Service
N
Because a low resistance pathway has been provided,
the large amount of current that flows along this pathway
Lightning
will cause a breaker to trip and the circuit to open.
electrode
Sign without Ground Fault Protection
= ground fault current path
Service
N
Because of the relatively high resistance of an earth ground,
Lightning
electrode
only a small amount of current will flow through the lightning electrode.
This small current flow will probably not be enough to open the circuit.
and possibly a fire hazard if the current level is high enough.
Panel
N
Panel
No ground
N
connection
No path for fault current
As a result, the sign case will become a shock hazard
September 28, 2004
Sign
Ground fault
Lightning
electrode
Sign
Ground fault
Lightning
electrode
19
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