Electrical Conductance of Carbon Fiber Tubes - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: February 03, 2015, 04:25:47 am
content: Anyone have experience with electrical conductance of carbon fiber composite tubes? I have some on hand, purchased from Tap Plastics in the US, and was thinking of using them for construction of some milliameter probes for my own use. The tubes are about 6 mm OD with very thin walls. Maximum voltage would be 240 VAC, mains source.Checked ohms with a multimeter (Extech 310) and probes a few mm apart on the side of the tube, and it read infinite. Also read infinite ohms when probes were placed on cut ends. Subjected a piece to 240 VAC (mains source) with large clips spaced about 2 cm apart, and there was no current flow (200 mA range). Smashed a few segments with hammer and anvil and tested the shards with an ohmmeter; again, it was infinite ohms.Yes, I've seen reports on the Web and youtube videos that show that some carbon fiber composites are very conductive, but the tubes I have appear to be an exception. The carbon fibers are embedded in a resin. The most common resins that are used in carbon fiber composites are epoxies and phenolics, both of which have good insulating properties. Don't know which resin is used in the tubes that I have. Main worry is that if the resin deteriorates, then the exposed fibers may be conductive. Though my tests indicate they should be OK, my inclination is to be a wuss and use tubes of some other composition.Mike in California

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2015, 11:35:13 am
content: Can't say I'd recommend it...If looks are the reason, why not use ordinary insulating plastics and wrap it with that "fake carbon fiber" print the automotive nuts use to look cool?I haven't measured any myself; I have seen carbon fiber composites listed in McMaster Carr's catalog which make no claim to electrical performance other than "not insulating". Good enough reason for me to not use it electrically.Tim

author: coppice
author_rank: Super Contributor
timestamp: February 03, 2015, 12:14:00 pm
content: Quote from: T3sl4co1l on February 03, 2015, 11:35:13 amI wouldn't trust the fake carbon fibre stuff to insulate well without testing it thoroughly with some abrasion. You get a nasty surprise sometimes if you assume that black pigmented materials insulate.

author: calzap
author_rank: Frequent Contributor
timestamp: February 04, 2015, 06:41:08 am
content: What I'm trying to build is a dedicated milliammeter for measuring current leakage between the secondary side of bench isolation transformers (120 VAC in and out) to the mains phase (hot), ground or neutral. Yes, mains ground and neutral are supposed to be at the same potential, but that assumes building wiring is OK. The concern is that a decrease in insulation resistance in the windings or other fault could render an isolation transformer less effective as a protective device. My isolation transformers have GFCI (aka RCD) protection on both sides, which should provide a some measure of safety.GFCI's in the US typically have a trip level of 6 mA (UL943 standard). I would like to check current flows from the secondary to ground or primary-side neutral or hot below this level to detect a developing problem (and just plain curiosity). Leakage could be load and temperature dependent, so occasionally checking with a multimeter when the transformer is first plugged-in won't suffice. My isolation transformers have always read zero current from mains ground, neutral or hot to either secondary contact when I've checked with an Extech 310 multimeter (100 uA sensistivity). Saw a little sign flicker with the Extech, so tried an Agilent 1240. It measured leakage on the order of 30-70 uA depending on the combination of secondary and primary contacts compared. My checks have always been in a no load situation.The idea is to have a milliammeter in series with a fuse (250 VAC, 5 or 10 mA) between the secondary of the isolation transformer and mains hot, ground and neutral. There would be a selection switch, and in addition, a momentary switch would be in series for making the reading. I don't like the idea of leaving the secondary permanently connected to the primary even through a 5 or 10 mA fuse, although I can't really see anything wrong with it. Perhaps at some future date someone might replace the fuse with a much higher rated one. An internal, soldered-in-place 10 mA fuse in series with a 5 mA fuse in a fuse holder might be the way to go. Even if the 5 mA fuse were replaced with a 20 A fuse, there would still be 10 mA of protection.It wouldn't really matter which secondary lead was used with regard to getting some kind of non-zero reading on at least one switch position if there was a fault, but the magnitude of the reading at a particular switch setting would be affected by the location of the fault within the transformer.One problem is how to connect the wires. I ruled-out off-the-shelf three prong plugs because they would both have to be males, and when one end was plugged-in, the other might be electrically active. One idea was to make my own plugs but with spring-loaded retractable sheaths around the prongs. This might make it safe for me (maybe not), but could be a problem if someone else plugs both ends into the mains even with the 5 mA fuse. Confusing the plug that is supposed to go into the mains with the one that is supposed to go into the transformer output might cause problems too. So, yeah, I've given up that idea.Safest course appears to be hard-wiring the milliammeter (and fuse or two) between the secondary and a switch that can connect it to mains ground (i.e. the case) or mains neutral wire inside the case or mains hot wire inside the case. Each transformer would need its own meter and some modification or extension of the case. Original hope was to have a meter that could be swapped among transformers, but the connector problem seems to have eliminated that possibility.Another problem is knowing whether a zero reading on the milliammeter is because there is no leakage or because the fuse has blown. Having a check position on the switch that connects to a milliamp source might be a solution, but switching arrangement and power source would need careful consideration .If there are other ideas for continuously monitoring primary to secondary leakage of isolation transformers, please post them.BTW, the continuity function on many multimeters is quite capable of blowing a 5 or 10 mA fuse (how do I know?).Mike in California

author: LaurentR
author_rank: Frequent Contributor
timestamp: February 04, 2015, 07:07:13 am
content: You could use a leakage clamp meter. No physical contact so you don't need to worry about safety.Look up the Fluke 360 or its Yokogawa OEM version. I have a Hioki 3283 on order for debugging tripping GFCIs in my house. Will report when I have experience with it. Got the Hioki despite worse resolution because it has a monitoring output, so I can put a logging meter on it to capture over a period of time if necessary.

author: calzap
author_rank: Frequent Contributor
timestamp: February 04, 2015, 07:38:54 am
content: Fluke 360 is a terrific instrument and would work if switched and fused "shorting straps" were brought out of the case in order to use the meter clamp. However, at US$700 it's a bit much for $300 collection of isolation transformers. Parts to do what I propose to my work-horse transformer should be less than $100. Still, it's tempting to get yet another type of meter.Mike in California

author: LaurentR
author_rank: Frequent Contributor
timestamp: February 04, 2015, 09:46:56 pm
content: The Yokogawa (OEM for the Fluke) and the Hioki are in the $300 range, quite a bit more affordable.