Whats the term I'm looking for? Copper between buildings

[Guest Guest]

Whats the potential problem with running copper cables between buildings causing electric shock?

[localzuk]

From what I was told, its all about the difference in earth voltage between buildings. Also, if the buildings each use different phases, there is potential for issues there too.

[nicklec]

Think technically it may actually be 'Potential difference' but the problem is what is described above!

[soveryapt]

If you're doing a new run I'd advise looking into Fibre. Obviously gets rid of the issues you're talking about and can give you a better connection.

 

Ok, it's more expensive to put in place and you may have to upgrade your switches at each end if you've not got the facility there, but it allows for better expansion. Obviously, depends on how many computers your talking about running too I guess to make sure it is effective, or if there are plans for increasing the numbers etc.

 

To the question in hand though, if you're running between buildings there are always risks involved, but you can minimise these, such as by using surge protectors on the Cat5, but (and I've never really done this so I don't know for sure) often the cost of doing that can be nearly as expensive as for getting a fibre run in anyway.

 

In terms of the Earth differences, it's worth getting this checked by a sparky as if its a really bad setup it could have quite a huge difference which could lead to quite high voltage differences, which again can be got round through using fibre instead (we've just had some new Portable Cabins put on site, and all the networking is done over Fibre because of these very concerns by the LEA team that arranged the install (we literally have a cabin with 1 PC in that has a fibre run all of it's own, and there isn't even room in the cabin to put in another computer realistically. It has it's own WiFi should they want to use laptops or something in there though).

[Guest Guest]

Cheers people.

 

we are getting a load of huts while our school is rebuilt. Meeting today so I need to get "their" heads out of the sand and highlight every issue I can think of. I know one thing which will be suggested is "will it be cheaper to just run copper?"

[glennda]

I know one thing which will be suggested is "will it be cheaper to just run copper?"

 

not when they have to rebuild because on a electrical fire!

[soveryapt]

will be suggested is "will it be cheaper to just run copper?"

 

To do it safely I doubt there would be much difference in the cost of copper vs fibre (when you've got the surge protection etc in place). Worth checking figures yourself on the cost of such things to see if money could be saved, or more to the point what the end cost difference would be to do it properly.

[glennda]

Yep worth doing it properly - also remember that temporary buildings never tend to be very temporary - ours are still here and where put in 20 years ago!

[HarryMonkey]

The problem could occur when the two (or more) buildings are fed by different phases of a 3 phase supply. Should an electrical fault occur which then passes via the copper network cables, you have effectively joined 2 phases giving you 440v through the supply.

 

One other selling point you could use is that if you use fibre, it can be run through the same conduit as the mains feed, whereas if you use copper it will require a conduit of its own, as it is not permitted to run mains with anything else through the same enclosure, which will clearly involve more cost.

[plexer]

Are there any documented cases of mains voltage being fed down cat5 by faulty equipment?

 

Ben

[Jamman960]

out of interest - If different parts of a building use different phases could that also cause a problem?

[featured_spectre]

Copper will be an issue because of the different phases, even in the same building...if they wish to risk electrical fires, signal degradation and be outdated, then copper is the way to go.

 

Fibre has little risk, and in the long term will be cheaper

[edutech4schools]

what happens if lightning hits the copper?

[HarryMonkey]

I'm sure there are cases where this may have happened. However, these are there as precautions. Would you really want to be the first docemented case simply because it hadn't happened before?

[plexer]

out of interest - If different parts of a building use different phases could that also cause a problem?

 

Yes if you connect one cab to another using copper and the switches are powered from circuits on different phases you have the same potential issue.

 

NICEIC will not allow 2 power sockets on different phases that are within reach of each other to be in existence.

 

Ben

[Jamman960]

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?

[Jamman960]

Yes if you connect one cab to another using copper and the switches are powered from circuits on different phases you have the same potential issue.

 

NICEIC will not allow 2 power sockets on different phases that are within reach of each other to be in existence.

 

Ben

 

hmm this could be interesting - we have several examples of this along with a few inter-building cat5 runs although its all been in place for several years

[plexer]

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.

 

Ben

[HarryMonkey]

If a switch becomes live ... fire melts mains lead insulation ... then the copper network cable will be the conductor between the two switches.

[plexer]

Can't see how but there you go.

 

Ben

[nicklec]

Its the difference between the two earths outside of Bruce Willis visiting the school....

[garrya100]

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 )

Edited May 26, 2011 by garrya100
typo

[HarryMonkey]

So the IEE regs and all the theory I learnt when I did my electricians papers is all wrong then?

[garrya100]

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.

Edited May 26, 2011 by garrya100
typo

[maniac]

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.

 

Mike.