Protecting the DC Power Source While in the Field

The need to know the condition of the power source for any field day / portable station is a major priority. The requirement to know exactly how much current is being drawn by all of the equipment at any given moment during the stations operation is also paramount.

When operating portable, sometimes in the remotest of location, to suddenly loose power due to a battery failure, when that illusive Grid Square is on offer, is nothing short of a disaster.

Coupled with the above, is the disaster of all disasters, a short circuit in the equipment or its cabling that puts a “dead short” across the power source! That pungent smell of melting plastic is not something I wish to experience when operating in a field day / portable location.

Lastly that impossible scenario, when arriving at the appointed location with minimal time to setup and then applying reverse polarity to the equipment, was not something that I ever wanted to happen!

Yes I have been guilty of experiencing all of the above over my 50 plus years of amateur radio operation, but it is something that I was determined not to have happen with my field day / portable operation this time around.

The following examines the Power Distribution System that I have used for my Microwave Project to ensure, as far as practicable, the mistakes of the past will never happen again!

DC Power Distribution

To begin with, I was determined that all cables feeding every piece of equipment would have an in-line fuse in the DC line. Further, every cable would be terminated with a connector. In the case of all the microwave transverters, every power connector would have the same type of connector that is keyed to prevent reverse polarity.

Every Transceiver DC Power feed, while having an in-line fuse, has a different style of connector fitted to ensure that the correct fuses are used for each transceiver: (eg the current drawn by an ICOM IC7000 is greater than that drawn by a YAESU FT 817ND)

Transceiver DC Cable Harness

Lastly, the power distribution system must accommodate meters for Voltage and Current, an Overload Circuit Breaker, Input polarity sensing and be sufficiently robust in construction, to allow the unit to be placed in any convenient location.

The  images show the final DC Power Supply distribution system. The handles on the front of the system are important and are not there for cosmetic effect. They were actually placed to protect the meter faces from accidental damage!

Portable 12 Volt DC Distribution

The internal workings of the power distribution system are the essence of simplicity. A number of basic designs were considered and then rejected. In the end, the system has met following design criteria:

Inside the DC Distribution System

A single switch isolates DC Supply from all devices.

Voltage drop between the input and the output is essentially zero (the purists will know that is not true, as if metering is introduced into the line, there will be some voltage drop but in this case it is insignificant).

All connections to the distribution system are via plugs and sockets that are rated at the maximum current that could flow under peak load conditions from the connection.

Interconnection cables are rated at the peak load that could be required by the respective connections

Under “overload conditions” the circuit breaker included will not reset instantly, but will reset after a short period of time, thus preventing the dreaded “on/off/on” condition.

Polarity reversal  results in non operation, rather than the destruction of a fusible link or the tripping of a circuit breaker.

Metering is achieved using separate meters for Voltage and Current rather than a ‘multi-meter” style of operation.

Meter Scales are easy to read and not confusing to check the status of the DC Supply System quickly.

Footnote:

A safety warning when using portable generators in the field

As mentioned in another article I use a Portable HONDA 2 KVA generator to charge the batteries via a multi-stage charger when operating for extended periods in the field. I am never ceased to be amazed at the number of amateur radio operators who use a similar system but fail to protect themselves and there second operators from electric shock.

I use a residual-current device (RCD) positioned at the generator just in case. As an Electrician and an Electrical Engineer I have seen, at first hand, what can happen when someone, in most cases through no fault of their own, become connected to 240 volt AC. It is not a funny experience and depending on the situation it can be lethal. These devices can be purchased for a very small cost and add a level of electrical safety that all should heed!

For those who are interested, a residual-current device (RCD) is an electrical wiring device that disconnects a circuit whenever it detects that the current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCDs are designed to disconnect quickly enough to prevent injury caused by such shocks.

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7 Responses to Protecting the DC Power Source While in the Field

  1. Peter VK3QI says:

    Roy,
    I would be interested in which model of Honda 2 KVa generator you use in the field and which brand (type) of RCD you use. the reason being that we have found that the Honda EU20i generator has a strange configuration in that the voltage measured between A and E is 120 volts and N and E is 120 volts (and A to N is 240 volts). MOST RCD devices that you could plug into the output of the generator, such as those built in to powerboards or plug in line, will not function correctly on the EU20i (also the EU10i).
    Cheers
    Peter VK3QI
    .

    • vk4zq says:

      Hello Peter,
      Thank you for the interest in the use of RCD.
      The Honda units that you have identified (EU10i and the EU20i) are not designed to be used with an RCD as the risk of having their frame connected to earth is very small.
      Therefore the only risk that exists with these units is if a human creates a circuit across Active and Neutral. In this situation the result can be lethal and an RCD will not help!
      An RCD is designed to sense an out of balance current situation in the active and neutral circuit. The only way that this can happen is when another circuit is introduced that causes current to flow back to the source (earth leakage). In the case of the generators you mentioned I have seen an earth stake connected to them and this is when the whole system becomes dangerous. If a person touches the active conductor while earthed then the result can be lethal.
      I have also seen a number of other generating systems being used in the field that have an earth stake connected to them. When this is done situation becomes very dangerous indeed. In both of these scenarios an RCD of the in-line type work very well and have the potential to save lives.
      The system that I use is an industrial system made by Toolex model 591017 that has a TH8RCD installed during manufacture.
      I hope this all makes sense?
      Regards Roy VK4ZQ

  2. Owen says:

    Some thoughts…

    The issue here goes back to the design of distribution systems.

    Real world distribution systems vary, but most connect the neutral terminal to ground at ‘source’ because it provides a more reliable architecture for detecting and isolating the inevitable ground faults.

    Ground faults are detected and isolated by an overcurrent device, usually a fuse or circuit breaker. The overcurrent must withstand normal load current, but fails on the very large current that flows with most ‘hard’ ground faults, a low impedance connection from an active wire to ground.

    The RCD provides a measure of protection where a person’s body creates the ground fault. The RCD depends on the fact that ground fault current does not flow back via the neutral wire, and so the device can be made to trip at currents much lower than the overcurrent device protecting the conductors. Further, to be effective in preventing electrocution (death by electricity), not only does it need to operated at very low currents (typically 10mA to 30mA), but very quickly.

    Now, if you take a portable power source such as generator, or battery powered inverter, where the AC output is isolated (insulated) from the external metal of the device and from ground, the distribution system is different, The absence of the grounded neutral at source prevents an RCD being reliable protection since the RCD will only operate if there are at least two fault paths created, one from one conductor on the supply side of the RCD, and one from the other conductor on the load side of the RCD.

    To my mind, for RCD to be fully effective on a portable gen set:
    1. the neutral must be bonded to the protective earth conductor on the supply side of the RCD; and
    2. the protective earth conductor must be bonded to an effective ground electrode.

    Whilst (2) might not usually be done by a tradie who wants to power up a jackhammer for 5 minutes, it is probably worthwhile for a 24 hour field day operation considering the complexity of cables, power boards, equipment, grounded antenna supports and feed lines (no to mention it rains on field days).

    If (2) can be done, then even for a gen set with fully isolated AC output, a small distribution board containing a main switch, RCD, and neutral – earth link (source side of RCD) could provide the point for attachment of an effective ground electrode. Of course, you would want to check with the manufacturer to determine that is was permissible to bond external metal of the gen set to one of the AC output wires. If the gen set is indeed double insulated,(it should be marked) that should be fine.

    My understanding is that Workcover has specific requirements for gen sets used in a work situation (they must be fitted with a 30mA RCD).

    Owen

    • vk4zq says:

      Hello Owen,
      Thanks for the comments. I agree with what you have said. MEN systems were developed for all of the reasons you have mentioned.
      Also the 30ma threshold you identified is a well established reference for earth leakage.
      As you rightly identified, the design of the generator is the key. When products are approved for use in this country the NATA Labs get involved and their reports will ultimately determine whether the product will be approved for use in this country.
      Use of an MEN system in the field is ideal however there have been numerous incidents that have seen people hurt trying to implement these systems on generators that are not designed to be used on MEN environments due to their internal wiring configuration.
      From the issues you have raised it is obvious you are well versed in the complexities that surround this issue.
      Thank you for your input it is greatly appreciated and I hope by raising this issue people using generators in the field will be careful and think twice before modifying or tampering with type approved generating systems.
      Regards Roy

  3. Owen says:

    Some thoughts for owners of the Honda EU20i gen set…

    I did have a quick look at the user manual for the Honda EU20i gen set. the circuit shown for the U model (which is what should be sold in Australia) does not show grounding of the middle of the output as is shown in the other models, so the output may well be isolated, apart from possibly filter components in the inverter output.

    Capacitors from each AC output terminal or connected load to ground may create the illusion of a centre tapped output. The device may well show 120V a side to high impedance loads, but may not sustain that if much current is drawn to ground. (One of the mistakes electronics people make is to measure power systems with high impedance voltmeters.)

    If the output is truly isolated, then a plug in portable RCD box will not deliver all of the expected benefits as there will be no unbalance unless there is also a ground fault on the supply side of the RCD. The RCD box won’t hurt, and it brings an overcurrent device and multiple outlets, but it is unlikely to ever trip due to a ground fault on the load side.

    I would not tie one of the AC output terminals to ground, effectively making it a neutral, to use it with an RCD without express approval of Honda.

    Owen

    • vk4zq says:

      Hello Owen,
      Great work and thank you for the research, I am sure all who own one of the Honda generators will appreciate your efforts.

      Regards Roy

  4. Travis says:

    The Honda 20i can easily be fitted with RCD protection thus allowing it to be used on construction sites. Plug in RCD 4way outlets should not be used with portable generators because non of them are fitted with an MEN link standard. A good electrician should be able to fit an RCD to a generator, adding the MEN link and if required remove the very rare centre tap connection from the earth pin. I found a great electricial company in Brisbane called gensafe that have a kit for the Honda 20 and 10 that works a treat.

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