Doing a commercial 70volt dist audio system. One tech told me I need to ground the shielded speaker cable with the emt while another told me that I didn't need to. So who's right?

I'm running the 18-2 inside the same conduit as my coax+power for the 24ac cameras, so I decided to use twisted pair shielded speaker cable that comes with a drain wire and 100% aluminum foil.

Shouldn't have to. Never had to myself.

Yeah...we do MANY 70v installs an never have done that or seen anything even suggest that. I have never used or seen anyone use sheilded 2 cond for 70v

So you can run standard sheathed speaker cable side by side with the cctv video for 100-150 feet all the way back to the head end and it doesn't interfere with the surveillance (or HDTV component video)?

I spoke to a tech from a national distributor and he told me that I should run the 70v in another conduit because it's considered high voltage. That's not going to happen because it takes a lot of effort to set up the emt, and so we have to be efficient as possible, that is using the same (1/2 and 3/4) conduit for both speaker and camera wire. All of the runs will hit several main junctions and there's a 2" conduit that leaves this box and so the wire will be all PILED up in these 2" trunks before they hit the rack location. There's no plastic, all of the cables will be inside of closed steel boxes or emt.

"tech from a national distributor " yeah that's a good one! Ask him why 1st off running a shielded 2 conductor in a 70V system somehow dictates noise rejection. Then ask him how exactly your are supposed to ground the shield and to what. Does he expect you to strap shield to ground at the 70V amp? Well while I can't understand how that will do you any good in that application, go for it! I couldn't even guess how many commercial 70V jobs I have done over the years but I have never done that or even have anyone suggest it. As far as running it with video in the distances your talking about I doubt the 70V speaker line will be the cause of noise, more likely the distance and or the cable itself. I don't think the shielded cable will cause you any problems so if it makes you feel better go ahead, couldn't hurt!

First off, I hope it wasn't me that said that.

My feeling is that preamplified signals require shielding, and that amplified signals generally don't.

I have used twisted speaker cable in similar scenarios without any issues!!!

what needed the proper shielding here is the video cable, so I hope you used something good, like Belden.

My guess would be that you could tie the shield to ground at the equipment for some rejection but you could also be setting yourself up for some other bizarre anomaly because that cable was NEVER designed to be shielded

It is true that there is some confusion about whether 70V audio is high or low voltage because the peaks are more like 90V. Most of the EEs that I know in the business contend that it is low voltage, but just barely. Remeber, a phone line can kill you in the right conditions and it's those conditions that the NEC is concerned with, not wether your install will have noise.

Best bet if EMT was truly required would have been to run it in different conduits and bill the client accordingly. It's not your responsibility to keep it cheap. It's your responsibility to make it work and make it legal.

I just scanned through the applicable sections of the '02 NEC, and shielded speaker wire was not directly addressed, but: the conduit itself must be grounded, and ground wires within conduit are supposed to be bonded to the conduit, typically at both ends as well as wherever accessible at boxes between.

Several years ago, we wired a new medical office (dermatology, actually) for distributed music, computer networking, and telephone. We used a 70.7v system, auto-transformer volume controls, and shielded 18-2. I used the shield drain wire to bond the speaker and volume-control frames to a ground terminal on the amp.

Maybe there was no benefit, but there certainly was no harm.

We always jump the shields through, and connect them to the GROUND LUG ON THE 70V AMP! They wouldn't put a ground lug on a 70V amp if you didn't need to use it. 70V is enough that if something went wrong and it wasn't grounded you could get a good tingle from touching it. If it is grounded, the path through you is more resistance than the path through that shield back to the ground lug, therefore the electricity goes to ground, not you.

A ground on your electrical outlet isn't needed for something to operate correctly. It is only when there is a problem that the ground is necessary. This is exactly why surge protectors require a good ground, when there is a problem, the voltage needs somewhere to go, and it will take the path of least resistance.

Shielded speaker wire?? . 70 volt doesn't require a shielded pair. Some regions might require bonding or even continuous conduit. But shielding doesn't apply to a 70 volt audio system. You might have confused this information with "bonding" , but I doubt that also.

However, If you are running other signals including other pairs of 70 volt conductors in very close proximity to a 70 volt audio signal,, you should be aware of the fact that a 70 volt signal can induce energy into an adjacent pair of wires. So,,, never run discrete zones of 70 volt audion in the same bundle for a significant distance. 50 feet or more. An RF video pair should be OK,, and a CCTV copper braid coax won't have a problem , nor will a properly balanced line or mioc. level signal, but always attempt to provide at least a foot of seapration on any of these parralel runs in excess of 50 feet.

Tekno, part of the reason I used the shielded wire is that I also ran the CAT-5 and phone lines in parallel paths throughout the building, following the corridors, and my desire was to protect those runs from the 70v line. Plus, I happened to already have a 1000' spool of it.

that's only true with MOV based surge suppression. Series Mode requires no such ground path.

grounding out the shield won't accually hurt anything. if the wire has a drain just tie it to the chassis of the amp. also, the conduit is grounded. and since it's metal it'll do the same job as the grounding wire in the speaker cable.

if it was me, i'd do it knowing full well that it's not going to do anything noticable, but then it's there just in case.

Larry, it's an interesting issue. I recently had an electrician tell me that 110 volt AC electrical signals contained in EMT or BX are supposed to shield energy gnerated from within unlike Lumex etc. Both EMT and the outer sheath of a BX cable are considerably substantial and contain a lot of metal mass that could absorb the energy.

. In the reverse scenario involvong a balanced microphone signal, the shield around the inner conductors absorbs the external interferance and by the time it reaches the inner conductirs it is of equal intensity as it travels in the same direction towards the primary side of the balancing transform subsequently cancelling itself out inside the transformer. But I'm not qualified enough to say whether a shield around a 70 volt pair will prevent induction from eminating into adjacent data or tel conductors. I don't see how it possibly can effectively.

Data / Tel as you well know can be corrupted by such interferance, that while not preventing it from communicating, can reduce it's performance by causing packets to be lost as processors interpret the corrupt data. in the case of Tel. it might just be manifest in some noise, or even the presence of the 70 volt audio program.

With more and more platforms moving to data cable wiring protocol, I have really become much more sensitive to the concerns regarding proximity of these runs to all other signal conductors.

I once experienced a serious 70 volt interferance problem in a large hotel that had separate hallway and guest room zones. The rooms were onl;y to recieve emergency paging,, the hallways were to recieve emergency paging as well as full time background music. I rann all of these conducxtors together. There were four wings to the comlex ( over 200 rooms) Well,, needles to say,,, you could hear a faint bleed of music in all of the rooms which was being induced by the hallway zones. I ended up having to build a relay system that both shorted out (terminated) the 70 volt conductors going to the guest room speakers as well as disconnecting these wires from the amplifiers so as not to cause the amplifier to see a dead short. When a page was activated, the relays un shorted the conductors, and re connected them to the amplifier channels allowing the page to be heard in the rooms. Prior to this,, I spent at least a day looking for a crossed wire not ever suspecting induction.