It’s not just “Ground is ground”
People familiar with electricity frequently accept the idea that “Ground is ground,” i.e., that in a house, especially with a grounding system that complies with the NEC or CEC, all points of the grounding system are at the same voltage.
The NEC/CEC support this impression by requiring that no AC current pass through the ground wiring, except under narrowly defined exceptions: fault conditions (a line-ground short or leakage), or the action of surge protectors, sending the surge currents into the grounding system.
Without lightning, in a properly wired house, this impression is correct.
However, with nearby lightning, or lightning which may attach to wires that come into the house via other paths (modes 2, 3, 4 of Figure 1), lightning can generate large currents in the house ground system.
For the long grounding wires in many real installations (Figure 5 and Figure 6 below), the voltage drop in the wire can be much larger.
For the examples shown in Figures 5 and 6, with a 3,000 A surge (10% of a moderately strong lightning pulse), with a 3 µs rise time, and a 30 foot (~9 meter) long ground connection between A and B or C, the voltage developed in wire A–B is ~10,000 V!
This voltage difference between different points in the grounding system is called ground potential rise, abbreviated as GPR. It is inevitable whenever large lightning surge currents flow through the grounding system of the house.
Ground Potential Rise within a Building
Figures 5 and 6 show simplified circuits for TV sets connected to a CATV utility. The only protection required by the NEC/CEC is a grounding block that connects the cable sheath to ground, where the CATV cable enters the building. A similar diagram would be valid for small-dish satellite receivers.
If the grounding block were replaced by a telephone (primary) entrance protector (NID), the circuit would be valid for a fax machine, or a PC with modem.
This is enough voltage difference to flash over most ordinary insulating barriers in the equipment. If this happens, the equipment will probably be severely damaged.
NOTE: If the NEC-required grounding is NOT present (i.e., if the entrance grounding block is connected only to an unbonded ground rod, water pipe, etc.), the situation is considerably worse, and may lead to fire or other damage in the house itself.
If the CATV, satellite, or phone cables do not enter the building near the service entrance, the only effective way of protecting the equipment is to use a multiport protector, as shown in Figure 1.
Multiport protectors (Figure 3) eliminate damage due to ground potential differences by using voltage limiting devices or a direct bond to reference together the signal wires and the AC wires when the voltage differences exceed safe levels, typically a few hundred volts.
Multi-port point-of-use protectors (also called plug-in protectors) normally consist of an AC protector and one or more signal-line protectors, in a single assembly, designed to be installed near equipment that connects to both AC and signal lines (Figure 4).
These protectors serve three purposes:
- The AC protectors normally have lower effective surge limiting voltage than the panel protectors and also might protect against sustained AC overvoltage.
- The signal line protectors normally have lower surge limiting voltage than the primary signal protectors and might also protect against voltages (such as AC voltages from accidental contact with power lines) which are be too small to be stopped at the primary signal protector.
- The grounds for all the protectors are connected (bonded) so that intersystem voltages are minimized.
Under lightning conditions, large voltages can be developed between, e.g., phone, CATV and AC grounds, and these voltage differences are frequently the cause of lightning damage.
It is important to realize that the multiport protectors usually do not significantly reduce the GPR between point A and point B.
In most cases, the impedance of the signal wire to the equipment, plus the impedance of the AC wiring, is much greater than the bond impedance between A and B.
So the vast majority of the incoming lightning surge current flows through the A–B ground bond, and exits the house via the grounding electrode, as the NEC/CEC writers intended.
If the voltage from A to B is 10 kV, and the voltage between the signal and AC connections at the equipment is only a few hundred volts, the remainder of the 10 kV must appear within the AC and signal cables, divided in proportion to their impedances.
So it is perfectly possible for an AC or signal wire to have 5 kV or more between its two ends, for the short time that the lightning current lasts.
Because of the short duration of the current, even small wires will usually not be damaged by these relatively small (a few hundred ampere) residual lightning currents. The action of the multiport protector, though, generates an additional GPR disturbance.
The voltage between the AC (green wire) ground at the equipment can be several kV different from the voltage at point B.
If the TV set is connected to other equipment that is independently connected to B by AC wiring, that voltage difference will appear across the other equipment, and may damage it.
Fig. 6 explanation – Ground potential differences within a building under lightning strike conditions: How down-line TV sets get damaged.
With a 3,000A surge rising in 3 µs, and a 30 foot ground bond (A–C), ~10,000 V develops between A and C. Even with a multi-port protector (D) for TV1, the ground voltage at D is conveyed to TV2 by the coaxial cable, resulting in an 8,000 V potential across TV2, which will probably destroy it. A second multi-port protector as shown in Fig. 5 is required to protect TV2.
Figure 6 shows a very common improper use of multiport protectors that does not fully protect against lightning damage because of this effect. One (AC + Coax) multiport protector, D, has been used in an attempt to protect two TV sets.
The installer assumed that the coaxial protector in D would remove the lightning surge, and any TV sets downstream would be safe without further protection. That assumption has limited validity for the voltage difference between core and sheath of the coax cable.
But it is totally wrong in describing the ground potential differences!
If the protector and TV1 are near the cable entrance at point A, most of the GPR at point A will appear at the protector D and TV1. But with no protector on TV2, the full 8000 V potential at D is conducted to point E, the input of TV2. The 8000 V difference between point E and the voltage at B, the connection of TV2 to the service panel, will damage TV2. To protect TV2, a second multiport protector located at TV2 is required.
If this is not done, equipment can only be protected by multiport protectors, located at the equipment being protected.
Surge protection alone is not sufficient to protect equipment. Inter-system bonding is also required.
Ground Potential Rise for Equipment Outside a Building
Equipment mounted outside a building is vulnerable to GPR damage because it is typically referenced to two grounds. Compressors, well pumps, spa and pool heaters, and other outdoor equipment are frequently mounted on concrete pads in contact with moist soil (see Figure 7).
In some cases, this pad can be a more effective ground than the building ground electrode. So the equipment ground is bonded to the pad ground, while the equipment line and neutral connections are referenced to the building ground.
For fast-rising lightning surges, the inductance of the equipment ground wire prevents the voltage at the remote pad from following the voltage at the building ground.
So there can be differences of tens of thousands of volts between the building ground and the pad ground.
Fig. 7 explanation – Equipment that has its own ground can be damaged by potential differences between two grounds. During a lightning surge into the ground electrode, the voltage rises by 750 kV for a 30 kA strike and 25 Ω ground.
The insulation between the motor coil and the frame/housing sees a significant fraction of the 750 kV developed at the building ground, and may break down the insulation of the motor, controls, or wiring.
The voltage at the motor coils is referenced to the building ground, because initially no current flows through the line and neutral wires, so the voltage at the motor follows the voltage at the building ground. The ground potential difference between the building ground and the pad ground appears between the motor windings and the (grounded) motor frame, and will flash over the insulation.
Lack of awareness of this cause of damage, and the remedy, is responsible for many cases of damage to outside equipment. The damage could be prevented by relatively simple additional surge protection, installed at the equipment.
Reference // IEEE Guide for Surge Protection of Equipment Connected to AC Power and Communication Circuits