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Topic: Will this setup cause a fire? (Read 4381 times) previous topic - next topic

tack

#15
Feb 05, 2013, 04:53 pm Last Edit: Feb 05, 2013, 09:08 pm by tack Reason: 1
The cable isn't rated for AC. You'll probably find leakage currents that it's not designed to withstand. This can cause stress in the insulation and ultimately a breakdown.

It's also not rated for current. With such a small CSA conductor you'll have excessive volt drop over a long length and also heating effects in the cable. Heat + PVC insulation = bad. The rating of the insulation will be less when hot and you risk melting it.

Transforming to 12V, rectifying and drawing 7A at the end means your 120V AC RMS current will be in the 0.7A+ region. The exact figures will depend on I2R losses, transformer iron and copper loses and overall efficiency.

Bottom line, No, it's not suitable for mains voltage like you are proposing.

If you need a run like that then use a 3 core cable of 1mm upwards CSA. With current below 2A I'd probably say 1mm-2.5mm flex extension cable would suffice, without consulting tables or doing a calculation. Laying in ducts or trunking will de-rate the cable too.

Nikarus

Im just wondering. Why cat5. Its easy enough for like $30 to get a nice 100ft 12v extension cord that will mroe then run your 12A adaptor way out there.
It's not like you need to be secretive or anything with what your trying to do. Are you like trying to drive the power through an ethernet cable thats already built into a wall or something?

David82

What other cable can carry that kind of power and data while being able to be bought locally in different lengths? Cat5 is very convenient. An extension chord would be good too but it only has 3 wires. I need power, RX, TX, and video signal all on one cable.

retrolefty


What other cable can carry that kind of power and data while being able to be bought locally in different lengths? Cat5 is very convenient. An extension chord would be good too but it only has 3 wires. I need power, RX, TX, and video signal all on one cable.


Part of most national safety codes is that one is not allowed to carry both AC power and DC signals within the same cable assembly, unless it's some specially build cable that is in effect two cables enclosed in one overall cable. This path you seem determined to keep on is with not a good one to pursue.

Lefty

David82

OK. I need AC power, video signal, and RX, TX all on one cable. What cable will do that?

oric_dan

Quote
OK. I need AC power, video signal, and RX, TX all on one cable. What cable will do that?

It took me 3 seconds to do this search,

http://www.google.com/webhp?source=search_app#hl=en&tbo=d&sclient=psy-ab&q=ac+dc+in+same+cable&oq=ac+dc+in+same+cable&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.41934586,d.cGE&fp=5888ed2a9af8b67f&biw=1005&bih=828

Delta_G

The thing I don't see coming up is interference.  Your 120VAC is going to step all over your video signal and serial comms if it is in the same cable even if it didn't catch fire.

PeterH


What other cable can carry that kind of power and data while being able to be bought locally in different lengths? Cat5 is very convenient. An extension chord would be good too but it only has 3 wires. I need power, RX, TX, and video signal all on one cable.


You're mixing data and video in the same bundle as power cables? That's going to need screening then even assuming you get around the conductor rating issues. UTP is not a good starting point for that.
I only provide help via the forum - please do not contact me for private consultancy.

WileyCoyote

#23
Feb 05, 2013, 11:26 pm Last Edit: Feb 05, 2013, 11:47 pm by WileyCoyote Reason: 1
Believe it or not, I actually do this type of testing for a living, so let me assure you - CAT 5 cable will carry 7A of AC current for approximately 2.5 seconds before fusing.  30A will last approximately 65ms before fusing, and 60A will last approximately 13ms before fusing.

I will not state how much current can be 'safely' carried by CAT 5 cable for your own safety...

Forgot to answer the second part of the question...you asked why would 120V cause the CAT5 cable to fuse when 12DC does not:

DC current will use the entire cross-sectional area of the wire to conduct current.  I do not know what the DC current rating of CAT5 cable is, but if you say you can carry 7A DC through it, I assume the cross-sectional area of 26 AWG wire is sufficient...

With respect to AC current - as frequency increases, conductors begin to exhibit "skin effect," which is that the current actually travels on the outer surface of the wire and not through the center of the wire.  As frequency gets higher, the skin depth gets smaller, so you are effectively reducing the area that the current is flowing though, so at 120Hz, you really don't have the equivalent of 26AWG copper wiring any more - you have something less (I don't know how much less, unfortunately).  At extremely high frequencies, the entire current path may be only on the surface of the wire itself.  30 Megabit Ethernet signals are very low voltage and very low current, so the amount of copper needed to transmit the data is very small, so why then is CAT5 cable 26 AWG??

The answer is simple - the telecommunications company that is sending service into your home has engineered their protection mechanisms to ensure that the most dangerous voltages your telecomm lines will ever see is 120V.  They also know that if you should get 120V on your telecomm lines, the equipment that your telecomm lines are connected to will have secondary voltage-limiting and secondary current-limiting protection.  In short, they know you have sidactors and fuses in your equipment.  Fuses are designed to carry up to 20% above their rated spec for up to 15 minutes, so a 2A fuse will carry up to approximately 2.4A for a full 15 minutes before fusing.  Anything more than that, and the fuse operates almost immediately.  When that fuse operates, the current is interrupted and your telecomm lines don't fuse and catch your house on fire...so, that CAT5 cable has to be rated to carry up to approximately 2.5A for 15 minutes...

Hope this helps.  :)

David82

Pretend like you have a device that draws 12V 7A at most, receives TTL data, and sends back video data. It needs to operate at varying distances, say, 6-100ft. It needs to be connected with one cable. How would you do it? Do you cop out and run an extension chord for the power and a telephone/CAT5 line for the other data, or do you get smart and figure out a solution that meets the requirements?

modeller


It needs to be connected with one cable.


Why one? Where did this requirement come from?

PeterH

The calcs above showed that with cat5 cable in order to stay within the cable's rated voltage and current you could carry the power but it would require using all the conductors. I wonder whether you could get away with using fewer conductors if you used solid core cat6 augmented cable? That has thicker conductors and would have a significantly higher DC current rating. Since you're carrying video and TTL data down the same cable I'm pretty sure that cross-talk would render AC completely out of the question so I assume you would transmit it using DC, and keep the DC voltage within the specs for the cable which probably means 50-60V maximum. Cross-talk issues may still kill you with UTP, though. You really don't want high currents running parallel to unshielded cables.

Again, this implies transforming and rectifying the power at the supplier, and doing DC-DC conversion down to 12V at the receiver side. It seems to me that the cost / complexity added by that would outweigh the convenience of being able to use standard network cable, but I don't know how interested you are in issues other than the cable itself.

You haven't mentioned where these cables would be used. Can we assume that they would be for permanent installation in a warm dry environment?

You might be able to get away with ethernet cable if you are willing to make enough compromises, but it doesn't strike me as the right cable for carrying this much power or that combination of signals. You really need a cable designed to carry the current and voltage you need, plus separate shielded cables for the video and TTL components.
I only provide help via the forum - please do not contact me for private consultancy.

David82



It needs to be connected with one cable.


Why one? Where did this requirement come from?
Because customers would prefer to buy and string up one cable vs two side-by-side.

retrolefty




It needs to be connected with one cable.


Why one? Where did this requirement come from?
Because customers would prefer to buy and string up one cable vs two side-by-side.


I figured this was for your own use. I shouldn't make such assumptions.  ;)

That's different, the customer is always right. In that case just present to and have your customer sign off on a waiver of all fault and liability for your work and run it all through your cat5 cable. Be sure you have your attorney draw up the document as some liabilities cannot be transferred or waived.

Lefty

Quick5pnt0





It needs to be connected with one cable.


Why one? Where did this requirement come from?
Because customers would prefer to buy and string up one cable vs two side-by-side.


I figured this was for your own use. I shouldn't make such assumptions.  ;)

That's different, the customer is always right. In that case just present to and have your customer sign off on a waiver of all fault and liability for your work and run it all through your cat5 cable. Be sure you have your attorney draw up the document as some liabilities cannot be transferred or waived.

Lefty


Waivers aren't going to help you when somebody gets killed. If anything it will show you knew it was dangerous and did it anyway.

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