Specifically, grounding for the DC circuits, and why is equipment being damaged?
It’s been 6 years since the last article on this blog about grounding. No damage from lightning (knock on wood) all these thunderstorm seasons later, is a good testimonial to the lightning ground. However, lately a few devices have been destroyed or damaged not by lightning but by something else. Considering there have been no thunderstorms the damage probably has been caused by voltage spikes from somewhere. But from where? A ground loop? A relay with no diode? One inadvertent source of a ground loop has been discovered. The negative side of the DC power bus has been connected to the Single Point of Ground since 2017. No noticeable problems have been observed due to this, probably because of luck.
Dereck Campbell, KF5LGW, has a good explanation of why connecting the two together is a bad idea. Note that Dereck is talking about an AC to DC power supply and the remote system has only a DC battery supply. Read around the references to AC line in the explanation below, please. Pay attention mostly to the right hand side of Dereck’s drawings.
The objective is to isolate AC and DC power systems and remove your radio equipment from the ground loop by relocating the ACEG to the same point the shack uses. To understand what is happening, let us draw the circuit out and see what is happening.
FIG 3A shows how 2-wire and 3-wire systems are incompatible. Look closely at the 2- wire system chassis (ground) and DC Negative circuit conductor and the same conductors bonded together inside your 12-volt radio equipment. That is not compatible with 3-wire systems. As you can see the red arrows result forcing normal DC operating current on ground conductors.
The result is DC flowing on equipment grounds, coax shields, and GES. Look what happens when I induce the same AC Line-to-Ground fault inside the DC power supply in FIG 3B. Follow the fault current paths like before. One path is on the ACEG conductor like you planned, and the other unplanned path goes through your radio equipment needlessly and can cause significant damage to your radio equipment and coaxes.
Take note. The 14 AWG ACEG conductor from the breaker panel to the wall outlet can be 30 to 100 feet in length. The unintended secondary path created is through your radio equipment using a 6 AWG conductor of approximately the same distance going back to the same point as the 14 AWG. They are in parallel. Which of those two parallel paths do you think will carry the bulk of the fault current? The 6 AWG or 14 AWG? All you need to know is how Ohm’s law works; no math required. Isolate the 2- systems, and you eliminate the problem!
Lost in a ground-loop creates two more issues. One a minor annoyance generating RFI/EMI. You have a piece of wire (you radio ) bonding the two ground electrodes together with a common-mode current flowing through your EGP. The second problem can be extremely hazardous if lightning strikes nearby. Equalization current path is right through your equipment, acting as a piece of wire.
Now, look at Fig 4A and Fig 4B. I removed the bonding jumper hiding inside the DC power supply. Follow the red arrow current again. What happened? AC and DC systems are isolated. No DC is flowing on any ground conductors. Removing the bonding jumper broke the DC galvanic bond across the transformer inside your DC Power Supply. AC and DC systems are isolated, allowing you to interface your home 3-wire system to your radio’s 2-wire system. The DC system can be a Grounded System, but do not use your DC power supply chassis ground because the transformer’s primary side is part of the AC system, including the chassis.
Why Is Equipment Being Damaged?
One possible explanation is the DC negative bus being grounded but that hasn’t caused known issues for many years. Could the damage be from “back-emf” from an inductance? When a relay is activated and then eventually deactivated there is a voltage spike from the coil which is back-emf caused by the inductance of the coil. A diode across the coil will short-out that back-emf voltage spike. There are several high amperage relays in the remote station that have large coils and large voltage spikes. Those relays all have diodes installed and it is unlikely those relays are the cause of the damaged equipment. What has changed lately is the West Mountain Radio 4005i power controllers have been replaced by a KMTronic 8 relay unit and a DC power strip. In the past, each device was powered individually by a port on the 4005i’s. Instead, now several devices are lumped together and get their power from the DC power strip through a relay on the KMTronic. Power to that strip is turned on and off by the KMTronic unit. Some of the devices are in the shack and others are out at the end of long runs of power cable. In-the-shack devices are in parallel with those long cables on the DC power strip. One tuner is 100 feet away and one security camera is also 100 feet away. That long cable may have enough inductance to induce a back-emf voltage spike when the KMTronic deactivates it’s relay. The plan is to add 1N4001 diodes across the long cables and then to add 14 volt Zener diodes across the inside-the-shack devices. It is hoped the diodes will provide protection from spikes and stop the damage to equipment. Or isolating the DC negative leads from the green-wire ground bus (single point of ground) will. Updates to come.
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