Presumably if you were to run an inter-building cat5 line in and attatch a poe powered access point to the other end you wouldn't need to worry? obviously this wouldn't be great for a more perminent solution but for short term portable cabins with few machines in it wouldn't be an issue.
The same would apply to digital phone systems where the phones are all powered from the pbx?
It came to light in our new workshop where the was a new supply installed but there was an existing socket from the old supply still in place, unfortunately they were different phases thus leading to a 410v potential across them.
I just can't see how you are going to get the issue over 2 switches connected via cat5.
Sure if you stick a screwdriver in 2 mains sockets on different phases.
If a switch becomes live ... fire melts mains lead insulation ... then the copper network cable will be the conductor between the two switches.
Can't see how but there you go.
Its the difference between the two earths outside of Bruce Willis visiting the school....
The problem with switches being on different phases being connected by Cat5 copper has nothing to do with the fire risk. The transducer on each end of the copper with blow long before enough current can flow to create a fire risk.
Even if you connected a length of Cat5 cable directly across two phases between to buildings the copper would blow like a fuse before a significant fire risk occurred. There simply isn't enough copper in the cable to sustain the current levels needed to create a fire risk even with a significant earth fault.
The issue with the potential difference between buildings (which always exists) is the small currents generated by the PD creates additional noise on the cable. With enough PD between to buildings the induced noise will actually swamp the data signal hence no data stream.
Even small currents/noise caused by the PD between buildings can cause bit errors, hence re-transmition, and a slow down in data through put, despite a lot of network traffic.
Optical fibre being either glass or plastic is an insulator, thus no current can flow regardless of the PD between the switches.
The other benefit is that fibre can go to 10G whereas copper's effective limit over any distance is in reality 400Mbs.
Garry Anderson (AS Dip Elect Eng if it helps to show I know what I'm talking about )
Last edited by garrya100; 26th May 2011 at 01:22 PM. Reason: typo
So the IEE regs and all the theory I learnt when I did my electricians papers is all wrong then?
No, not entirely, the regs are designed to help stop the induced noise, and minimise the shock hazard, which can still exist if a mains cable is connected some how to a data cable, either by accident, human error or equipment failure, and can still exist even if the transducer in the switch has blown up as a result of being connect to the mains.
A fire hazard and shock hazard are two different scenarios.
You can still create a shock hazard if a building has a major earth fault and a cable connected to that system is being handled by some poor individual some distance away. A Cat5 cable doesn't contain enough copper to sustain a fire hazard, but certainly can sustain a current of 30ma, which is all that's required to kill someone!
In that situation it's only the impedance of the poor individual on the end of the cable that limits the current flow till the breaker trips or the copper blows.
Before somebody brings up lightning strikes again, if you remember the theories of electro-magnetic induction, it's the sudden motion of the electrical field produced by the lightning strike that creates the voltage rise in the cable, and a lightning strike miles away can generate enough voltage in a copper cable to destroy a switch transducer.
Again optical fibre is immune to lightning strikes.
I can tell you it still goes against the grain designing systems that run data cables in the same conduit as 11Kv lines even though it's perfectly legal using fibre. Copper has to be a minimum of 6 feet away and shielded and then have its own earthing system.
In a correctly installed system, with solid earths, the shock hazard is minimal, and the cable itself can't support a fire hazard, but there will still be PD between the buildings that can't be easily eliminated, though it can be minimised. The induced noise can still beat you, even though you've done everything correctly!
I can add more theory regarding voltage drops/current flow/cable CSA if you wish, but it may be a little heavy for this forum.
Last edited by garrya100; 26th May 2011 at 02:21 PM. Reason: typo
With modern RCD's on circuits, the risk of this actually happening is very very small. I know for fact that pretty much any school will have computers connected to different phases wired back to a single cabinet, how could you ever be sure that there isn't.
The issue of potential earth differences over cables only applies if the buildings earth is independant of the main building. Our science block for instance is a completely stand alone building that has a 3phase supply from the main building, but a completely seperate earthing system, therefore there is potentially a difference between the two buildings earth. Most mobile classrooms are fed via overhead cables on a single phase including an earth from the main building, which means you can quite happily run copper to them - the only risk being the tiny tiny issue of the cable picking up static from nearby lightning - this can be negated by putting a supressor at each end of the cable.
I've had mobile classrooms at several schools I've worked in with copper cable feeding them, and never had a single issue.
Not Wrong, just very over cautious.
Think about it. If a switch had mains voltages present on a port, then thats dangerous anyway. Even if it where connected directly to neutral or earth, there can still be enough voltage to give an unpleasant shock at the end of a long cable. They are designed to be connected to cables up to 100 meters, so differences in earth potential is going to be a common one.
Switches and other similar equipment are designed so this can't happen, even with a significant fault. There will be physical isolation of wiring, as well as transformers or a well designed switch mode psu to separate the high and low voltage circuits. This is assuming you don't buy really cheap no-name stuff from ebay which probably won't have this level of protection.
In order for 2 phases to be connected via the switches you would need to very bad faults in two separate bits of equipment. Even then, the fuse should blow at one end breaking the circuit. Failing that, the cat 5/6 cable will overheat and melt, which should be ok, as long as you haven't buried it in kindling, or run it through a petrol station. Proper installed cabling should be low smoke and self-extinguishing.
The amount of things that would have to go wrong for this to become dangerous make it an unlikely scenario.
You will almost certainly have problems with noise and packet loss though. So you do still want to use fibre for these kind of long runs between buildings. There might also be problems with rcds tripping etc.
You should get a qualified installer to do external cabling, as there are other importan issues running copper between buildings. Direct lighting strikes being one, properly sealing the entry point into the building is the other main one. A qualified installer would almost certainly recommend fibre, I'm sure.
Last edited by Chris_Cook; 26th May 2011 at 02:16 PM.
Ahh! Petrol stations! I've had to do them too. Sheilded cables in conduit, buried in 1ft of sand, minimum 3ft underground, with vapour pits no more than 30ft apart, voltage suppression arrestors no more than 50VAC...... I hope I never have to do another one!!!
BTW the current flow in the neutral conductor can have a large impact on the PD if the buildings have separate earthing systems. One thing you learn in engineering is that earth is not a particularly good earth! Hence the MEN system that has multiple earths tied to the neutral to establish a solid earth. Bear in mind that the earthing system used in power transmission is designed for safety, not low noise.
It's to make sure that when there is a fault, the fuse will blow, or the breaker will trip.
Reduced sized neutrals in 3 phase systems, and different earthing conductor sizes, while perfectly legal and adequate for a power system can have large impacts on the PD between buildings. A five volt PD between buildings is no issue for a power system, and the induced currents are small, but for a low voltage data system the difference is huge.
Last edited by garrya100; 26th May 2011 at 02:54 PM. Reason: typo again!
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