![[Field Day Power Distribution]](fdpower.gif)
Just because its temporary doesnt mean it has to be intermittent!
by Rob Spence, N1ICB
You have just taken over for your night shift at Field Day. The sounds of crickets are harmonizing with the heterodynes; the aroma of a hot cup of java mixes with the sweet smell of summer flowers; the Q rate has climbed past 45; the background drone of the generator is reassuring and suddenly there is sputtering sound followed by quiet and darkness. Wheres the flashlight! Uh oh, they didnt tell me how to restart the generator!
You are working Qs just fine and all the power goes away (hmmm, but the familiar background drone is still there)! You check at the generator and find a tripped circuit breaker and a sheepish team member who says but I only plugged in my small coffee pot.
Sound familiar. This was typical at our clubs Field Day back several years ago before we decided to get a little more serious about Field Day competition.
I am a member of the Billerica Amateur Radio Society (BARS) here in Eastern Mass. We compete in class 5A and had managed to reach 5th place. This got us talking about the possibility of getting to 1st place.
It didnt take much heavy thinking to conclude that we dont get a lot of Qs when the power is down so when next years planning got underway, reducing downtime for power was high on the list. We figured that we could guestimate the generator run time until it needed fuel and then go around and notify the station ops before we would shut down the generator. Since we could anticipate the fueling time it was pretty easy to get everything ready in advance; get the cover off the gas can and install the pouring spout and get a light ready so you could see while the generator was off. That way each fueling only took a few minutes. That was the good news. The down side was that it still disrupted the stations for about 5 to 10 minutes - try and get all the stations at a Class 5A site to agree on a time to shut down!
Well, it must of worked. Our score was lots higher and we moved up from 5th to 3rd place in our class. Of course we did improve the antennas and overall operating skills too.
This confirmed our hypothesis that the power system was one of the key areas for us to look at in order to increase our score. We determined that there were two areas to work on:
What follows is a discussion on what our club did about eliminating these concerns.
Getting the downtime for fueling to a small number might seem to really solve the problem (and Im sure that for some, it will) but when every Q counts, any power outage adds up. For example, we refuel about every 4 to 5 hours or so. This means we would nominally have 5 outages that each cause about 10 minutes of disruption to our 7 stations. This adds up to over 5 station hours of downtime. Thats a lot of lost Qs.
We have 2 kinds of critical station loads on the power system, radios and computers. We also have lots of other loads like lights, keyers, rotators and, of course, the required coffee pot. The ancillary loads are insensitive to power outages for the most part - basically, if the power is OK for the radios and computers, it is good enough for the rest of the stuff. We did some simple experiments and determined that the power supply in the radios had sufficient energy stored to ride out a short outage of less than a half second. We also found that we could take advantage of modern notebook computers for logging as most of them will automatically use the internal battery if the AC power is removed.
With this knowledge in hand we plunged ahead and built a switch panel that connects one of two generators to the load. In our operation we use two panels with a primary generator connected to each and an auxiliary generator that can be connected to either panel as needed. The panels can switch a 30A 240V line under load even though the typical load is more like 10 -12A. There is an interlock on the switch panel which prevents a switch over from proceeding if the auxiliary generator is not on-line. There are indicators that show the status of the power sources and also which source is currently active. An elapsed time meter serves to track operating hours between fueling.
In operation we use a log sheet for each switch panel. When we start up a generator we log the time of day, the elapsed time meter reading, and a comment. During operation we monitor the fuel level in the generators. When a unit is getting low we connect the auxiliary unit to the switch panel and start it up. While it warms up we update the log sheet with the time, the meter reading, and the estimated fuel level. Then we switch the load over to the auxiliary unit and shut down the primary. While the primary is down we top off the fuel and check the oil, adding some if needed. Then we start the primary, let it warm up, update the log, and switch the load back. This keeps the stations on line and makes sure we have time to do proper maintenance on the generators.
Here is a case where a little advance planning plus some training makes all the difference. We designed a set of power cables that distribute power at 240v to boxes containing pairs of duplex outlets located near the station locations. This provides a duplex outlet at 120v on each side of the 240v line. A special adapter inserted in the line near the generators provides small current measuring loops - one each for the two hot lines, the neutral and the safety ground. Using a current probe we monitor the load on each side of the line and in the hour or so before the contest begins, fine tune it by moving a station or two from one side of the line to the other as needed. As night approaches we can carefully add lighting loads to the circuits with ample capacity. If someone comes up with something during the contest that needs power we can check the system load and determine if and where to add it. We produced a one page handout of power system guidelines that is placed at all stations and shown to all people installing equipment. It asks the other team members to check with the power crew before plugging in something new to give us a chance to make sure it wont overload a circuit.
There is something about Field Day that attracts bad weather. We almost always get some rain and have had hail, lightning and strong winds too. We drive ground rods for each station just as we all do at home, right? At the generators we drive two rods and connect them together with heavy cable. A safety ground wire is run from each generator and switch panel to the ground rods. To further increase electrical safety and reduce the chances of a power outage we add some weather proofing to all the unsheltered connections. For simple in-line connections we wrap them with several turns of plastic wrap and secure the ends with twist ties. At boxes where the duplex outlets are we just stick the whole box in a 2 gallon size plastic bag and again use a twist tie to secure it. These dont make em completely water tight but they do protect well against a day or so of rain. We have removed the plastic after a heavy rain storm and found the connections nice and dry.
Well, we had one person this year who noticed one of our switch-overs :-) He was looking at a light and saw it flicker. We have used this power system for three years now and have had three field days with non-stop power for the whole contest.
During setup there will, of course, be times when power tools of various types will be needed. Just check with the power crew before using please so we know who is currently depending on power. Also, please let us know when you are done. Thanks!
During the contest please ask first before connecting non-station types of equipment. At times we may be running near circuit limits and additional unexpected loads could trip circuit breakers and disrupt station operation (something I am sure we all want to avoid). The power crew will do its best to accommodate any reasonable request, even if we need to run an additional circuit to do so.
Logical schematic for power distribution at BARS Field Day, 1995