High-voltage issues
Greg Gimlick [email protected]
It's been a busy period since the last column. I recently returned from a trip out west to see my new (and first) granddaughter. I flew with the pilots at Cherry Creek State Park again this year in Denver. It's great to hang out with those people.
I've received quite a bit of mail regarding high-voltage setups, so I thought it would be a good time to respond to some of those questions and concerns.
Fire in the Hole!
It's easy to become distracted at the field; people want to talk about your airplane while you're connecting things, etc. I'll admit that I enjoy a nice conversation in the pits, but when I cover events I try to remain cognizant of how distracting I can be with my camera and interview questions. Unfortunately, I'm not always that "aware" when I'm at my own field and in my comfort zone. At big events, I'm more focused for fear I'll screw up in front of a crowd.
I received some photos from a good friend of mine showing the results of being distracted while getting ready to fly. This is not a man who is new to electrics by any means; he is a well-known expert in our field, but a small distraction nearly ended in disaster.
He had two 4S packs connected in series to power his 8S airplane. The connectors are color coded and he was connecting a black to a red between the two packs, leaving a red from one pack and a black from the other to connect to the ESC.
During final arming, the gentleman was asked a question, which distracted him. He grabbed the two wires from the pack and connected them, forgetting that he had already made the series connection so he shouldn't be connecting any unmatched color connectors. This created a dead short of his 8S pack.
The flash was instant and impressive.
Because the pack burned the connectors so completely and instantly, he was able to grab the wires and pull the pack from the airplane before additional damage was done. Surprisingly, the packs did not ignite beyond the initial flash.
Fortunately, the contacts in the Anderson Powerpoles quickly shorted out and burned off so the circuit was broken. That left the burn marks and melted plastic to deal with, but he wasn't injured.
How can this happen? Easily!
Most of us who use larger-size LiPos use multiples to make up our 8S, 10S, or 12S battery packs by connecting them in series. This was the case in my example. There are some simple steps we can use to avoid a similar situation; here are just a few to keep in mind:
- Stay focused! Ask visitors to wait a moment while you complete the task, then you'll be glad to talk to them.
- Standardize your method of keeping connections identified.
- Be consistent in your methods across all airplanes and helicopters.
- Double check everything before making the connection.
- Check it again!
- It doesn't matter what method you use, but use something.
I'm guilty of not following all of my own advice, but I'm trying to get better after seeing the results of easy mistakes. On my largest 12S setup, I have color-coded connectors that are also numbered with tabs. This means I always plug the number one plug into the other number one plug, the number two into the other number two, and so on, until it's all connected. If the numbers don't match, they don't get plugged in, regardless of the color of the connector.
High-Voltage Danger
I think there are wars being fought on the Internet over what constitutes "high voltage" in our airplanes and helicopters. I stay clear of all the arguments, but it does make for some interesting reading. It also confirms my thought that anyone with a keyboard can appear to be an expert on a discussion forum. Be cautious and do your research before accepting anything you read there.
In the "old days," AstroBob of AstroFlight fame would say he'd never make a motor/ESC system for more than 36 cells (we were using Ni-Cds at the time) because of Underwriters Laboratories (UL) requirements and various interpretations.
Throughout the years, this was debated and various interpretations of the UL guidelines have seen this 36-cell or 50-volt limit go by the wayside. Now we regularly see systems all the way up to 15S LiPos. My fully charged 12S system is more than 50 volts when I first connect it.
I'm neither an electrical engineer nor a lawyer. I'll leave the particulars of what constitutes high voltage to them, but I have read and researched more UL guideline papers than I care to think about and there is a common theme across several of them. Here is an excerpt that appears in several different papers regarding high-voltage DC circuits:
"... a circuit fire hazard is considered to exist if either the voltage is greater than 35 volts or the current is greater than 0.2 amperes."
I pulled this particular one from UL-6500, but it is in several publications, so I assume it is the basic guideline. There are two important things to notice here: voltage and current. The hazard doesn't come from a combination of both parameters; it's either/or.
I can't think of a system we're flying that doesn't draw more than 0.2 amps, so in essence, everything we do is a fire/safety hazard.
Are we unsafe? That depends upon us. If we consciously do things in a safe and educated manner, then the answer is "no." If we become complacent or careless in our thinking, then "yes."
Many people don't think about DC voltage being dangerous because it is so much lower than the AC voltage we're accustomed to at home, but it has been clearly documented that it can be equally dangerous. The best system is only as good or as safe as the person using it, so be aware and be careful.
Anti-Spark Devices
This is getting into Red Scholefield's end-of-the-world territory, but because I've received so many emails after showing my big 12S, 99-inch airplane, I'll mention it briefly. The anti-spark device I used in the Robinhood 99 is from Xtreme Power Systems and sells for a few dollars.
This device doesn't remain connected as others do; it is plugged in for a few seconds to slowly charge the caps on the ESC, then disconnected. As soon as you disconnect it, you plug in the final battery connector and all is well. If you wait too long, the caps will self-discharge and you'll get the spark when you connect your battery.
The only precaution I take with this particular one is to insulate the bullet connector when I disconnect it so it doesn't accidentally short against anything. It is a positive lead going through the connector, so anything it might touch that is negative would produce a short.
There are several places online that explain how to make your own and they are a simple device. I'm lazy and these are inexpensive enough that it doesn't seem worth the effort for me to make my own. I think Red described several months ago how to determine the requirements of the resistor and make it.
Taking Off
That about covers it for this issue. Regardless of whether you're flying 3S-size setups or monster 12S setups, you need to be careful. Don't be afraid to ask someone to wait a minute so you can finish your task. Then you can visit with him or her knowing everything has been done correctly.
SOURCES:
- Xtreme Power Systems
[email protected] www.xtremepowersystems.net
Transcribed from original scans by AI. Minor OCR errors may remain.
