Interference ?

[Comhairle]

Hi all. I work in a venue in Dublin and there are plans to build and underground rail line underneath our theatre. Obviously with electric trains there would be quite a substantial EMF surrounding the motors on the train. Instiinctively I don't think it should be a problem but I know the hospitals are causing a bit of a fuss about some of their gear being interfered with. Does anybody work in a venue with a similar sceanario? I'm slightly concerned about the PA buzzing every few minutes when a train passes. I think the tunnel will be 25 meters underground.

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[paulears]

Bearing in mind the West End in London is criss-crossed with tunnels, I suspect that interference, if present, would have been a hot talking point since, well, a long time! I know I've heard a few grumbles about mechanical rumblings from beneath - but don't have any absolute answer. Interesting point is on ownership of the land underneath your building. is there a magic figure below which people can bore away without your permission. I understand BT pay you a leeway if they go across your garden even in the air, so maybe below has rules too?

[Dmills]

I would worry far more about the vibration then the electricity.

Low frequency structure bourne vibration is next to impossible to filter except at the point of generation.

 

IIRC St Georges hall in Liverpool has a line under it that was put in on rubber sleepers to reduce the vibration transmitted.

 

At 25M down it may be less of a problem then we have with the cut and cover job just outside the front of our building.

 

Regards, Dan.

[djw1981]

Remember London Underground and Merseyrail electrics are DC electrics. Thius may or not be a factor.

[Bobbsy]

Working in TV in London, I've had to deal with studios above underground lines and also adjacent to an overground rail line once.

 

EM interference was never an issue but, as others have surmised, we did have to spend a lot on acoustic isolation against the inevitable LF rumble. Alas, while this was possible on new-build studios, I don't think floating floors and "box within a box" isolation would be practical for an existing theatre.

 

Bob

[dbuckley]

The only electrical issue with trains is common to overground and underground; you need to keep the train rails isolated from "ground" otherwise the return traction current gets into every bit of wire for miles around, since electricity uses all parallel paths available to it...

[KevinE]

you need to keep the train rails isolated from "ground"

 

Someone needs to tell the guys in charge of the Trams in Manchester & Blackpool, then! Last time I put my foot on one I was sure it was pretty well embedded into the tarmac!

[bruce]

We've got a railway tunnel (proper railway, 25kV AC stuff) running under several of our buildings at work. Not a theatre, but there are dozens of lecture theatres above (all with PA, induction loop, radio mics etc), and a radio station. Never known of any audio problems due to the electrical impact of the trains.

 

But you can often hear - and sometimes feel - the trains going by underneath, especially on the ground floor.

[dbuckley]

Someone needs to tell the guys in charge of the Trams in Manchester & Blackpool, then! Last time I put my foot on one I was sure it was pretty well embedded into the tarmac!

Look more carefully.

 

I lived through the powers that be debugging the grounding errors in the Croydon tramway which was playing havoc with phones....

[Malcolm Gordon]

I understand BT pay you a leeway ....

Helpful pedant mode: I think you may mean "wayleave".

[paulears]

accepted with extremely good grace because the idea interested me and Google does seem to need the correct word to produce meaningful results - thanks for that!

[KevinE]

Look more carefully.

 

You run your trams with an earth return, or not? The ones in Manchester have earthed rails and an overhead live conductor. People can put their foot on the rails without danger, providing the tram isnt too close.

[Comhairle]

Thanks for the replies. I'll sleep easier now!

[AndyL]

Thanks for the replies. I'll sleep easier now!

I don't know about in your particular area, but the hospitals concerns might also be related to vibration. I heard that a hospital near me recently had their eye surgery ward relocated form an older ground floor building with a nice solid concrete slab floor to an upper storey in a brand new building with suspended floors that 'move' (boing, boing) every time someone walks or moves anything anywhere in the building, or when a truck goes by. Not good when you are dealing in micro millimetric surgical matters apparently. They had to move back to the old building, although, allegedly only for a while. ;)

 

As to what any large subteranean electrical current flow might do to any types of medical imaging systems they might use...

 

I you are interested in the subject of electrical supplies to urban transit, take a look at the arrangements they use in the French city of Bordeaux: Ground-level power supply ...very snazzy.

[dbuckley]

You run your trams with an earth return, or not? The ones in Manchester have earthed rails and an overhead live conductor. People can put their foot on the rails without danger, providing the tram isnt too close.

Absolutely. But like I said:

 

Look more carefully.

Those rails are isolated from ground by an insulating membrane or barrier. This is to stop traction return current getting into other wiring and pipework thus causing interference or corrosion.

 

I'm not making this stuff up, leakage of traction return current from the rails through the soil has been a problem since day one of railway electrification. For example, here's an abstract of a paper on the subject entitled "Overview of stray current control in DC railway systems":

 

In DC rail transit systems, the running rails are usually used as the return conductor for traction current. This arrangement mainly focuses on economic considerations, since it does not require the installation of an additional return conductor. Low resistance between the traction return rails and the ground allows a significant part of the return current to leak into the ground. This is normally referred to as leakage current or stray current. The amount of leaking current depends on the conductance of the return tracks compared to the soil; and on the quality of the insulation between the tracks and soil. The stray currents represent serious problems for any electrified rail transit system. The corrosion problem has been a major concern to the railway and other parties involved since the early days of DC railways. The stray currents create or accelerate the electrolytic corrosion of metallic structures located in the proximity of the transit system. This causes metal pipes, cables and earthing grids laid in the ground near the tracks have a much shorter life which is high importance in regard to safety and economy. Hence, great efforts and research have been carried out to control stray current in DC electrified rail transit system. Stray current control is essential in these railway transit systems where the [return] rail insulation is not of sufficient quality to prevent severe corrosion to the rails and third-party infrastructure. Good control of stray current is also of direct benefit to the operational and safety aspects of the DC electrified railway systems; it could reduce the rail touch voltage. This paper provides an overview of control schemes currently applied to the DC traction electrified rail transit system.

Niasati, M. Gholami, A: This paper appears in: Railway Engineering - Challenges for Railway Transportation in Information Age, 2008. ICRE 2008. International Conference on.