Online assistance for electrical trade people in New Zealand and Australia Login  |  Register  |   Forgot Password
Assistance for electrical trade people
 

 

 

 


Click here to send Forum Admin a pdf document for publication on this Topic

Documents must be less than 200k in pdf format

Posted By Topic: TPS wiring

Hendrix1
Jul 31 2019 09:18

Apart from the obvious Electrical good code of practice of installing flat TPS cables clipped to surfaces without twists and a known problem of twisting cables is the effect of over bending/stressing the cables cores does anyone know of any other problems/effects of having twists in the flat TPS cable.
   

Wiredone
Aug 02 2019 23:07

No, if anything having twists in your cable is a good thing, as it cuts down on inductance. But obviously dont go twisting your TPS up lol. The odd twist or bend will have no significant effect on the cables performance or durability.
   

pluto
Aug 03 2019 11:17

As above and provided that the minimum bending radius is NOT smaller than that pemitted by as/nzs 3000:2007 clause 3.9.6 refers.
   

evanh
Aug 03 2019 14:05

The twisting in a twisted pair will increase the inductance. Designed to balance out the capacitance of having two parallel wires. I don't know the maths but I think it reduces the capacitively driven resitive heating losses in an AC signal.

The same mechanics also creates an effective shield against injection of differential noise.

   

dlink
Aug 03 2019 16:25

Evan, thats alittle DC v AC too.
   

evanh
Aug 03 2019 17:18

As in DC is just as happy without the twists? Yeah, I guess that's true.

   

SymonS
Aug 04 2019 11:37

"The twisting in a twisted pair will increase the inductance"...

Good thing the original poster wasnt asking about twisted pair cable then...
   

evanh
Aug 04 2019 20:44

The mechanics applies to any paralleled wires. I highlighted where twists are used intentionally is all. It was partly an explanation of why it will create an increase in inductance rather than a decrease.
   

JonoRTC
Aug 05 2019 13:04

EEE checking in here.

It's a little different for something like TPS and twisted pair given how they're arranged, but the physics is the same.

The twists don't so much increase the inductance as make it predictable. The total loop area between the two wires is what determines the inductance. Twisting the wires together should actually lower the inductance, on average. It also makes sure that area doesn't change as much as the wire moves, etc.

Twisting also normally increases capacitance between the wires by keeping them closely arranged and increasing the overall length of wire for the same length of cable run. Not enough to make much of a difference at mains frequencies, though, and it wouldn't make a difference in TPS where the wire positions are already fixed. It might increase the wire length slightly for the same distance run, I guess.

As mentioned above, it also adds some robustness to noise by allowing electric and magnetic fields to balance out over the length.

Intentional or unintentional twisting in TPS would have a pretty negligible effect under any situation that would seem reasonable during install, I'd suspect.
   

evanh
Aug 07 2019 18:35

Thank you Jono. I'll remember that. We keep learning.

There was another topic, see link below, that I was interested in that I couldn't easily figure out - What's your take on dbuckley's write up about capacitive "leakage" causing the lighting of fluoros held up underneath high voltage lines rather than just the sheer voltage?

I'm thinking that, while AC does have additional capacitive IR losses, the capacitive action doesn't produce leakage in itself and that the leakage is all down to voltage and dielectric properties of the air. ie: The fluorescent tube would glow just as well under a pure DC line.

https://www.electricalforum.co.nz/index.php?action=more_details&id=1532948396
   

AlecK
Aug 08 2019 08:31

The fluo glow won't happen with d.c because it's not a capacitive effect, it's the effect of a fluctuating electromagnetic field.
Any discharge type lamp will glow if you put it into a fluctuating field, as the field excites the particles which then interact with the phospor coating on the discharge tube.
Works even better using a VHF or UHF transmitter; that's partly frequency and partly proximity, as like all field effects the inverse square law applies.
   

JonoRTC
Aug 08 2019 12:36

No worries, Evan. We certainly do keep learning and this place is a gold mine of real-world information and interesting stuff for someone like me who is registered as an EEE but without the real world experience that is so critical in so many cases.

That's an interesting consideration. I'm not actually all that sure on the fundamental physics for the tubes themselves, but I'd say that I wouldn't think they would glow under just a DC electric field because I can't see how that would cause a load on the DC lines corresponding to the power that was converted to light. The static field isn't transferring energy in any way, so it would have to appear as a leakage current, which wouldn't make sense.
An AC field, on the other hand, has a reasonably high 'current' flow due to the constantly changing electric field resulting in the changing electrostatic charges. Having something that is 'breaking down' and conducting in the presence of that field would then look like a corresponding resistance added to the impedance of that line.
It's definitely possible to excite things like tubes with DC (laser tubes work like this) but that's an actual 'breakdown' mechanic where the electric field is enough to cause direct conduction current to flow through the item, rather than what you'd see in an AC field which doesn't necessarily have current flowing, as such, but does have electrons moving to a different spot with the application of the electric field. I guess a way to think about it is that current makes electrons move permanently (although they may just move back and forth in an AC system) while capacitance makes electrons move temporarily (and they return once the voltage is removed).

There will definitely be some leakage current whenever there's an AC voltage and any capacitance. There are a couple of ways you can end up with leakage currents, one being resistive losses (like surface tracking across a dirty insulator etc.), the other being through any capacitance. Resistive leakage would work for AC and DC systems, while capacitive leakage is only when you have a varying voltage, so AC systems.
Capacitance itself is basically just a measure of how much total current is required to bring a structure up to a certain voltage. In a DC voltage situation, that current flows as the voltage first rises and then reduces to zero. It's fundamentally a measurement of how many electrons can crowd onto a given surface and be pushed off an opposite surface when there's an electric field (voltage, more or less) present. A Farad is defined as the capacitance which needs 1 coulomb (which is an 'amp-second' or 1 A for 1 second, 0.5A for 2 seconds, etc.) of current to be charged up to 1 Volt. A 1 Farad 12V DC capacitor with 1A going into it constantly would have its voltage go up by 1V per second. In the same fashion, if you draw 1A out of the capacitor, the voltage will drop by 1V per second.

When you've got an AC voltage across that capacitor (or a mostly DC voltage with an AC component), you're going to have an AC current going through it in the same fashion, because you're essentially charging it up and discharging it over and over.

So yeah, you end up with a very measurable current flowing through the capacitance which just looks like a normal current, except that if you have a power meter that can measure the phase of the current vs the voltage, it will be reactive power, rather than real power. In an ideal capacitor with no losses or resistive component, it will look purely reactive and won't actually have any real loss associated with it in the capacitor itself. In reality, there are still all the IR losses you mention because that leakage current is still flowing in conductors etc. and that produces losses.

Actually, I suspect you might mean that the capacitive action itself doesn't produce loss as such, rather than doesn't produce leakage?
   

evanh
Aug 09 2019 04:11

For me, leakage means electrons flowing to earth from the lines. So, for example, heating of wires is a loss, but not a leakage.

I see AC charging and discharging action as a net zero outcome in terms of the capacitance itself. Only the resistance of the wires produces heating loss from AC charge/discharge, ie: crap power factor.

Alec,
Damn, that might throw a spanner in the works.

Hmm, I do feel a desire to build something now ...

   

JonoRTC
Aug 09 2019 14:35

Right, I see what you mean.

Well, that capacitance will still cause a current to flow from phase to neutral or phase to earth. It won't have a DC component, of course, but the net effect is still that current will flow, which causes the associated loss mechanisms. There will be some extremely minor dielectric loss, but the majority would be resistive losses due to the extra current and, as you say, 'crap power factor'.

The leakage current itself can have any phase relationship being from capacitive, to resistive or inductive, depending on what is causing the leakage. Either way, it's still a leakage current, to my mind, as it's not a direct load current necessarily.