Nuts and Bolts of
I have been designing quite a few small off-grid solar
power systems for end-users who want to run livestock water pumps,
remotely-located flow meters, emergency call-boxes, etc. Many times grounding is
the last item addressed, so I’d like to present a little Grounding 101 class for
you who have courageously ventured into the arena of off-grid solar and wish to
properly protect your investment.
What s Grounding? Grounding is essentially the sending of
electricity into the ground. Grounding is used to control dangerous
over-voltages, making equipment and personnel safer by providing a low-impedance
path (oath of least resistance) for any unintended voltage that is
on the metal parts of equipment, i.e. a “zero volts” condition on all metal
parts. Soil composition and environmental conditions determine soil resistance
around your ground, including moisture content (more is better), mineral levels
(more ions is better), and temperature (warmer is better).
What Should You Ground? You will want to ground any
electrical system that has the potential to over-voltage enough to cause damage
to personnel or valued equipment. For instance, there’s no need to ground a
small flash light — it won’t hurt anyone, and it’s cheap and easy to replace.
You will, however, want to ground your —48VDC 2000 amp telecom switch. In the
case of solar-powered
Systems, the panels are frequently mounted on the top or
side of a pole, and can make quite an attractive lightning rod. Often that
system will include a radio antenna mounted on a ten to twenty foot high pole.
Equipment performance can also be affected by intermittent interruptions that
can be caused by a poorly-designed grounding sys tem, so best to do it right
from the start.
How Should You Ground? For process control and
telecommunications circuits, the vast majority of Grounding and Bonding
specifications recommend a resistance to ground of 5 ohms or less. The earth
itself is a poor conductor of electricity, but given enough patting area,
resistance can be brought quite low. This is often obtained through the use of
deeply-driven ground rods or other earth-penetrating metal piping. Ground rods
are typically copper or copper-clad, and sunk at least 8’ into the ground. Metal
water pipes (not PVC) are often used if they provide sufficient patting area.
Never ground to metal gas pipes, as they could explode when supplied with an
ignition source such as a lightning strike.
Take a look at an off-grid solar power system, such as the
ones we make to power flow meters at remote gas pipeline sites (see the Solar
Off-Grid Customer Application Profile on the next page). It has one 4.5 watt
solar panel and a 12V 100-Ah battery backup. The greatest source of danger from
over-voltage at these sites is a lightning strike to the 20’ high antenna. The
entire power system is essentially metallic and well-connected — a strike to any
part of it would severely damage all the electrical components if not
sufficiently protected. A 5/8”, 8’ copper-clad ground rod is driven into the
ground, a copper ground rod clamp firmly fastened to it, and #6 stranded copper
ground wire run into the steel battery cabinet and securely attached. All of the
other power equipment is wired inside the battery cabinet. Since the remainder
of the system wiring is #12 copper stranded with an ampacity of 20, and the #6
ground wire has an ampacity of 75, electricity flowing through the
System from a lightning strike will flow through the more
accessible path — the larger-ampacity ground wire — and then straight to the
ground rod, to be dissipated into the surrounding earth.
The components for the grounding system are quite
inexpensive, and well worth the effort to install. If you lose equipment due to
over voltage, not only will you have the expense of replacing it, you will also
have the expense of your systems down time. Intermittent operation due to poor
grounding is frustrating and can damage components as well. According to the
Electrical Power Research Institute, “electrical wiring and grounding defects
are the source of 90% or all equipment failures”. The best course of action is
to eliminate the causes, starting
With proper grounding procedures and practices (NEC Article
250-5 1 and CEC Article 10-500).
energy, solar panels, photovoltaic cells, batteries, inverters,
power, electricity, energy