AC 1000V + loose termination = heat and melted sheath. - Page 1
Forum section: electronics / beginners

author: Strada916
author_rank: Frequent Contributor
timestamp: February 15, 2013, 08:23:36 pm
content: Ok I want to know if there is such a thing as volt drop in an AC circuit. Electrician at work found a heated and melted wire in one of the junction boxes. This wire carries 1000V at about 30A. (mining JB). The screw terminal it was terminated to was loose and therefore caused the wire to heat up and melt the plastic sheath. It also started melting the copper at the contact point.Now is this called volt drop, as I Know VD is caused by parasitic resistance/impedance, such as loose termination. The heat build up tells me that there is some form of impedance/resistance and therefore causing the wire to heat up can someone get me on the straight and narrow.Thanks.

author: IanB
author_rank: Super Contributor
timestamp: February 15, 2013, 10:16:14 pm
content: What's the puzzle here?There's a voltage drop in all circuits. The magnitude of the voltage drop is proportional to the current flowing and proportional to the resistance, so circuits with higher currents need thicker wires to lower the resistance. All wires, all terminals and all conductors have resistance and heat up when current flows through them. (Let's ignore superconductors.)When current flows it heats up the conductor it is flowing through according to the formulaW=I2R, whereWis the heat produced,Iis the current flowing, andRis the resistance.For example, if you had 30 A flowing through a resistance of 0.1 ohms you would produce 90 W of heat, which is about the same as a shining bright 100 W light bulb. It's quite a lot.In short, if you have high current circuits don't have loose connections. They start fires. If you have a loose connection in your home somewhere it could start a fire and burn your house down.

author: Strada916
author_rank: Frequent Contributor
timestamp: February 15, 2013, 11:01:19 pm
content: Is it still called volt drop in AC circuit? Where the wire and terminator melt? As the the sparkly looked confused when I said volt drop. He said volt drop is voltage drop in the wire it's self. I said I know as the wire is a resistor. But he still didn't get it that the heat was a product of parasitic impedance. Due to loose termination of the wire. Just want to know the correct term.

author: Chasm
author_rank: Regular Contributor
timestamp: February 15, 2013, 11:24:41 pm
content: Yes it it still voltage drop.Just because the frequency is not zero Ohms law is not suddenly invalid...

author: Rerouter
author_rank: Super Contributor
timestamp: February 15, 2013, 11:37:46 pm
content: you just have a voltage drop that varies as the line voltage varies, to perhaps simplify it for you, pick a single point on the sine wave, you can treat it as you would for dc analysis,

author: IanB
author_rank: Super Contributor
timestamp: February 16, 2013, 01:12:34 am
content: For design purposes you usually work out a voltage drop over wiring runs. The lost voltage depends on the current flowing, the wire gauge and the length of the run.You don'tdesignfor any voltage drop at a terminal block. You assume this will be zero (it will be essentially zero if the screw is screwed down tight on a properly prepared wire terminal).When you have a loose, badly made connection, then you have an accidental voltage drop. It's not called a voltage drop in design codes because it is not supposed to be there, but it is a "voltage drop" nonetheless.

author: SeanB
author_rank: Super Contributor
timestamp: February 16, 2013, 06:31:44 am
content: Any contact can have a voltage drop. You normally find loose terminals burnt because the loose screw causes poor contact ( basically now a high resistance as opposed to the required very low one) so now heat is generated at this point which then causes oxidation of the metals and a higher resistance, leading eventually to a burnt terminal, either the housing melting or the metal melting ( depends if it is plastic or ceramic housing) and eventually arcing which either disconnects the wire or causes a fire. Even at 30mA an arc can put considerable heat into the connection, I have had lots of Neon signs where the wiring burnt off for a considerable distance due to arcing, even though the transformer is a near perfect 30mA current source, though it can deliver up to 12kV open circuit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 16, 2013, 06:57:46 am
content: Quote from: Strada916 on February 15, 2013, 11:01:19 pmfrom one point of view. yes it is a "voltage drop", but the fact it occurs at small region (and length), so dV/dl or dV/dv (volume) is quite high, so is the energy dissipated at that particular small region. imagine you cook a food with the same amount of energy. the lesser the food's volume the higher the temperature will be, ie quickly heated up. high temperature will cause mechanical failure. its not necessary applies to only loosen contact, but also on a contigous wire with different diameters along the length, the smallest diameter will produce the greatest heat. the analogy to mechanical system is a notch in a straight bar under tensile force, that notch will fail first. and also it got nothing to do with AC, AC actually is the friend (lower RMS value), DC is worst at causing this. in your case, the heating is much more caused by DC (or RMS) rather than AC.

author: TerraHertz
author_rank: Super Contributor
timestamp: February 16, 2013, 08:41:53 am
content: I don't understand how you don't understand.Regardless of DC or AC, there's current flowing. There's resistance in everything (except superconductors) and so there is always an induced voltage drop across any circuit with current flow. Current flowing across a resistive path always generates heat, as I x V = Watts. The V is voltage drop, produced by I x R = Voltage.Substituting, it's I^2 x R = Watts. ALWAYS.The heat is just not normally destructive, since power distribution circuits are designed with thick enough copper so the energy loss (heat dissipation) is acceptable for the expected current. Still, it's noticeable, as high power circuits do get warm.Until something goes wrong; for instance a loose connection that develops a few ohms or more of resistance, at 30 amps. Say 2 ohms, that's 30A squared x 2 = 1800 Watts of heating at your screw terminal. Problem...Here are some pics of a similar problem. A water sterilizer on a ship, these cabinet feed throughs carried about 100A. But the connection tightness depended on the size stability of the plastic insulators. In addition, it's a DC system and the sterilizer needed to have the polarity swapped regularly. I suspect people were not tightening the nuts well after swapping connections over.It got warm, the plastic distorted, and so the connection got worse and worse. You can see the result. The last pic is of the replacement I made, where all connections depend only on adjacent nuts on the threaded rod. Also, using non-meltable insulators.

author: Strada916
author_rank: Frequent Contributor
timestamp: February 16, 2013, 11:19:04 am
content: Quote from: TerraHertz on February 16, 2013, 08:41:53 amMate I do understand, I just wanted to know the correct term. I understand the theory and why it happens. The sparky at work needs to go back to electronics school not me. I just want to know if "Volt drop" is the correct term to use when we are talking AC. thats all. Sorry to confuse anyone.Strada916

author: Strada916
author_rank: Frequent Contributor
timestamp: February 16, 2013, 11:20:46 am
content: Quote from: Mechatrommer on February 16, 2013, 06:57:46 amYes I understand all that, like I said looking for the correct term ie "Volt Drop"

author: Monkeh
author_rank: Super Contributor
timestamp: February 16, 2013, 11:22:50 am
content: Volt drop is a symptom of the problem. The problem being a high resistance/impedance joint. Volt drop does not in any way imply a hot or melting conductor.

author: G7PSK
author_rank: Super Contributor
timestamp: February 16, 2013, 03:46:13 pm
content: If you have a loose screw terminal in all likely hood you will have arcing which cause a lot of heating. You will certainly have a volt drop over the arc.

author: rsjsouza
author_rank: Super Contributor
timestamp: February 16, 2013, 04:17:15 pm
content: Based on your description (overheating due to loose connection), I believe the term you may be looking for is resistance heating. I foundthis reference(check section 14-9.2, subsection 14-9.2.3), but of course this is based on the United States code.I also thought about the term "arcing" due to a loose connection, which I think happens at a microscopic level when the oxide is formed. But in this case arcing may be more suitable to proximity and not to loose connections.

author: Strada916
author_rank: Frequent Contributor
timestamp: February 24, 2013, 10:44:10 am
content: Thank you all.

author: Nickk2057
author_rank: Regular Contributor
timestamp: April 21, 2013, 02:25:28 pm
content: one thing i do remember when i used a neon transformer is when i hooked it up to a 50W light bulb.... the farther away the arc is the brigbhter the bulb gets... the closer the dimmer.... and when it touches the contacts it gets way dim.... kinda interesting to know since we is talking about voltage drops here....