Newbie Question

[Guest grahamc]

Many thanks for all your replies and advice.

I will certainly use a fuse at the power supply end and also consider seperate cabling for the power supply. I don't know the current requirements of the camera system but the recommended supply is rated 1 amp. I assume that the camera probably draws 750 mA or thereabouts, putting it at the limit of doubling up a pair of cat5 wires (700 mA).

Thanks for all your help.

Graham

[dynamic]

Hi Graham,

Doubling up wires isn't really good practice.

As others have posted, the fuse is to protect the wires from overheating in the event of a fault. The worst case fault is usually a short circuit at the appliance (i.e. camera) end of the wire. In your case, this would result in the full 1A current of the power supply being drawn down the wire thereby exceeding its 350mA rating.

Notionally, doubling up the wires would give you 700mA capacity, but there's no guarantee that the wires would share the current equally. The amount drawn down each wire depends in detail on the resistances of the wires, and their terminations. You can probably expect very long wires to have equal resistance, but the way they are terminated often results in quite big differences in the termination resistance.

The real problem though is that your worst case fault hasn't changed: it's still any fault which results in all the current going down one wire. This could be caused by damage to the cable or the terminations. Also, in a permanent installation, there's no obvious way to tell whether the two wires are still connected. Would you check them once a day, week, month, year? Your system could run for years on one wire without anybody knowing until a short circuit develops, doesn't blow the 1A fuse, and the whole thing goes up in flames.

It may sound over the top, but overheating wiring really is a major cause of fires. If it were my home or business, I'd want the separate cabling that you've considered.

Best of luck with the project.

[Guest grahamc]

Hi dynamic,

thanks for your input into this. I agree with you that seperate cabling for power would be best. I am not sure how power over ethernet equipment can help since presumably the power is still going through the cat5 cable ? Is that true?

Hi Graham,

Doubling up wires isn't really good practice.

As others have posted, the fuse is to protect the wires from overheating in the event of a fault. The worst case fault is usually a short circuit at the appliance (i.e. camera) end of the wire. In your case, this would result in the full 1A current of the power supply being drawn down the wire thereby exceeding its 350mA rating.

Notionally, doubling up the wires would give you 700mA capacity, but there's no guarantee that the wires would share the current equally. The amount drawn down each wire depends in detail on the resistances of the wires, and their terminations. You can probably expect very long wires to have equal resistance, but the way they are terminated often results in quite big differences in the termination resistance.

The real problem though is that your worst case fault hasn't changed: it's still any fault which results in all the current going down one wire. This could be caused by damage to the cable or the terminations. Also, in a permanent installation, there's no obvious way to tell whether the two wires are still connected. Would you check them once a day, week, month, year? Your system could run for years on one wire without anybody knowing until a short circuit develops, doesn't blow the 1A fuse, and the whole thing goes up in flames.

It may sound over the top, but overheating wiring really is a major cause of fires. If it were my home or business, I'd want the separate cabling that you've considered.

Best of luck with the project.

[Guest]

I am not sure how power over ethernet equipment can help since presumably the power is still going through the cat5 cable ? Is that true?

Yes, PoE is basically just a standardised way of doing what you originally suggested. Some equipment is designed to be used this way though so can safely be used with power fed to it over a cat5e cable provided it is fitted in accordance with 802.3 guides.

[arfur mo]

hi grahamc,

always best to power locally if you can and many places will supply a 1amp 12vdc PSU for get the type with a powere lead rather than a moulded plug affair and place it inside a stout plasitic box tking precautions against water ingress if outside. you can still use the CAT5e for video signal, or if a new cable you can get a 'shotgun' cable which is RG59 coax and 3amp twin run mouldeded along side, but DON'T run mains on it as you may cause an unexpected, and dangerous surprise to the next guy who has not ingaged his 'smarts' before his cutter's.

its only 30 meters, and your camera is below 300ma @ 12vdc as long as you terminate properly and carefully, use the previous advice on wiring configuration, fit inline fuse at the power end for safety. i'd think you should be ok.

regs

alan

Alan no Trade prices in the public areas please

[dynamic]

PoE transmits the power at a higher voltage (typically 48V or 56V), so the current is lower. PoE devices are available which then convert this to useable voltages such as 5V or 12V at the appliance end.

[Guest]

(typically 48V or 56V)

Where have you seen 56V PoE devices? I believe to comply with 802.3af it's got to be 48V so it can be classed as SELV.

[dynamic]

Hi Lurch,

You're right that the SELV limit for AC is 50V, but remember PoE devices work with DC so SELV allows up to 120Volts (wiring regs 16th Edition).

Otherwise, it seems it's another case of product technology being ahead of the standards.

IEEE802.3af allows an output voltage from the PSE of 44-57V, though power should be limited to 15.4W. Phihong (http://www.phihong.com), for example, have exploited the voltage range by producing a unit which regulates right up to the 56V and "....is designed to give 19.6W while complying with IEEE802.3af detection and fault protection requirements." You'll note that they're not claiming to be fully 802.3af compliant.

So let me correct my earlier post in as much as PoE is available at 56V, but you're right, it's not typical.

802.11at (in development) defines PoE plus with the objective of achieving higher powers, see http://www.powerint.com/poe_ieee8.htm

[Guest]

Well, that's sorted me out then!

Oddly enough, I had a cpnversation with someone a bit ago and I had to point out to him that in fact 802.3 was for use with DC so his 'hombrew PoE device' was way out there in that it used AC. Obviously I've forgot I knew that!

[arfur mo]

Obviously I've forgot I knew that!

oh it comes with age mate, i do that all the time

the wife gives me those goo goo eye's, i get all excited but can't remember why

regs

alan