Douglas Krantz - Technical Writer - Describing How It Works
Go to the Fire Alarm Home Page of Douglas Krantz -- Describing How It Works
Go to the Fire Alarm Operation Map Page of Douglas Krantz - Technical Writer
Go to the General Electrical Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Description Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Installing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Maintaining Map Page of Douglas Krantz - Technical Writer
Go to the Fire Alarm Testing Map Page of Douglas Krantz - Technical Writer
Go to the Fire Suppression Map Page of Douglas Krantz - Technical Writer
Go to the Science Article Map Page of Douglas Krantz - Technical Writer
Go to the Writer Home Page of Douglas Krantz -- Describing How It Works

General Electronics

A second ground that isn't connected to the first ground on a building is a bad idea
When adding the resistance between ground systems, that's a minimum of 40 Ohms. Lightning is going to damage anything that is connected between both grounds.
Douglas Krantz -- Fire Alarm Engineering Technician, Electronic Designer, Electronic Technician, Writer

Does a Second Ground Rod Protect Circuits from Lightning?

By Douglas Krantz

Often times, for lightning protection at the signal wire entrance to a building, someone will drive a copper stake into the earth, but there's a problem. For the building's ground system, the main electrical wire entry already has a copper stake driven into the earth; a second copper stake driven into the earth for the signal carrying wires is a separate earth ground system.

Using two different earth ground systems for lightning protection is as bad as having no lightning protection at all.

Lightning and Ground

With two ground rods that aren't connected together, lightning puts most of its power into one earth ground connection or another. One ground stake is going to be affected with a voltage spike much more than any other.

Earth Ground Resistance and Voltage

Any earth ground connection has some resistance. As a best case scenario, the lowest resistance to the earth that most earth ground connections have is about 20 ohms.

This may not seem like much, but lightning has a lot of current (often it's over 10,000 peak amps). Using Ohm's Law of E (voltage) = I (current) times R (resistance), one can see that 10,000 amps peak current times a resistance of 20 ohms equals a total of 200,000 peak volts.

This peak voltage has a very short duration, but it's there.

200,000 Volts

When lightning strikes a building, the inside building ground system can have a spike voltage of 200,000 volts or greater. The whole building leaps to 200,000 volts at once; everything attached to the building ground system gets that same 200,000 volt hit. Because everything is connected to the building ground, the voltage between each device in the building doesn't change much.

Second Ground Rod - No Voltage Spike

As the building gets the 200,000 volt spike, the second ground rod holds the voltage on the other incoming wires closer to earth ground voltage potential. This voltage difference, between the incoming signal wires and the building electrical ground, could be 200,000 volts or more.
Was this
Yes   No

There is equipment standing between those incoming signal wires and building ground. Be it a fire alarm panel, a security panel, a cable TV amplifier, a CCTV camera, a telephone, etc., the only route for that 200,000 volts is from the building electrical ground, through the equipment, and into the wires being held to earth ground potential at the signal wire entrance. Something in the equipment is going to give.

That's not lightning protection.

Lightning Coming on the Wire Entry

Of course, if lightning strikes a power pole outside, or the PIV (Post Indicator Valve), or another building connected with signal wires, the same story can be told in reverse; but the result is the same: lightning damages the equipment.

Ground Bonding

This is why, according to the National Electrical Code (NEC), if an extra copper stake is driven into the earth, by using heavy wire, that extra copper stake needs to be connected directly (bonded) to all other copper stakes driven into the earth. That way, when the building gets its voltage spike, all earth ground connections will get approximately the same voltage spike, and there will be better lightning protection for the whole electrical system.


Douglas Krantz

Describing How It Works

View Douglas Krantz's profile on LinkedIn


Readers Questions

Short Circuit
Free Subscription
I'll Send You the
Fire Alarm

Get Short Circuit


How Does Class A Fire Alarm Wiring Work?-- Fire alarm systems save lives and protect property. Fire alarm systems also break down because... Read More

Just What Is a Signaling Line Circuit (SLC)? -- The SLC (Signaling Line Circuit) is another way of saying Data and Power Circuit. Along with added power to run the sub-computers and their input and output circuits, it's a computer data-buss ... Read More

How is a Buffer Relay Wired Into a Door Holder Circuit? -- Like a door stop, a door holder keeps a fire door open. When smoke is detected, the door holder releases, allowing the door to shut. The door holder looks simple and innocuous enough... Read More

How Does One Find a Soft Ground Fault? -- Normally, we think of resistance like that of a resistor. The amount of resistance is built-in; no matter what voltage is used to drive the electrical... Read More

Can a Magnet Really be Used to Test a Smoke Detector? -- Smoke detectors usually have two ways of being tested. Smoke (smoke particles in the air, or some sort of canned smoke), and magnets (the activation of an internal magnetic... Read More

Electrical Flow

On this website, most references to electrical flow are to the movement of electrons.

Here, electron movement is generally used because it is the electrons that are actually moving. To explain the effects of magnetic forces, the movement of electrons is best.

Conventional current flow, positive charges that appear to be moving in the circuit, will be specified when it is used. The positive electrical forces are not actually moving -- as the electrons are coming and going on an atom, the electrical forces are just loosing or gaining strength. The forces appear to be moving from one atom to the next, but the percieved movement is actually just a result of electron movement. This perceived movement is traveling at a consistent speed, usually around two-thirds the speed of light. To explain the effects of electrostatic forces, the movement of positive charges (conventional current) is best.

See the explanation on which way electricity flows at