Table of Contents
Advertisement
September 28, 2004
Lightning strike protection
A sign bonded to an earth ground has a means of dissipating the high voltage and current from a lightning
strike. The resistance of the grounding electrode should be as low as possible. However, damage can still occur to
a sign's electronic equipment from lightning voltage transients.
Though some surge protection is incorporated into a sign, to protect a sign from high-voltage lightning
transients, surge protectors need to be installed at the panelboards (see NEC Article 280 and 284).
Electronic equipment protection
A common cause for the failure of sensitive electronic equipment is the presence of objectionable current (also
called objectionable neutral current) on grounding and bonding paths.
Objectionable neutral current can be caused by:
•
Errors in installation wiring
•
Improper neutral-to-case bonds (illustrated below)
•
Equipment-grounding conductor used to carry neutral current — This situation arises when no separate
grounding wire is present when connecting power to a sign. NEC Article 250.32(B)(2) does permit a
neutral-ground bond to be used in a separate structure if all of the following three conditions are met:
(1) an equipment grounding connector is not run with the supply to the structure
(2) there are no continuous metallic paths bonded to the grounding system in both structures involved
(3) equipment ground-fault protection has not been installed on the common AC service
Adaptive does not recommend using the equipment-grounding connector to carry neutral current as permitted by NEC
250.32(B)(2) because it creates a potentially hazardous situation. For example, a future installer might connect cabling
Adaptive Explains
How can you tell if objectionable neutral current is present?
A true RMS microohm multimeter can be used to measure the voltage difference between the neutral and ground
conductors. Though a difference of 0V is ideal, the voltage difference should not exceed 0.5V.
20
Objectionable Neutral Current caused by Improper Neutral-to-Case Bond
Service side
Service
N
Because of an improper neutral-to-case connection (shown above),
Lightning
a shock hazard will be created because of potentially hazardous current
electrode
flowing on conductive surfaces like the sign's housing.
In addition, this current flow may cause electromagnetic interference
between the two structures and this could create a dangerous parallel current path.
AlphaEclipse 3600 Series A Sign Installation Instructions (9711-8001C)
= normal current path
= objectionable neutral current path
Load side
Panel
N
that disrupts the sign's internal electronics.
Sign
Lightning
electrode
Installation
Advertisement
Table of Contents
