100 Volt Audio, Band I or Band II?

David Stone said:

This is the problem with feature creep in BS7671 and the other standards

Works both ways. Been involved in looking at power over Ethernet in recent years ... and the issue there is the standards don't consider special locations, yet the use-cases people want for the technology definitely do feature special locations.

Hmmmmm ....

gkenyon said:

you simply can't carte-blanche run 100 V line cables with all SELV/PELV and assume it will be OK.

That I agree with.

But the guidance in BS7671 is not really appropriate - it is written by power people who think milliseconds are fast and microseconds are non-existent. high speed data, audio and pulsed power are very different disciplines, yes there is some overlap, but not a blanket cover-all.

Would you put an HRC fuse or an RCD on a 100V audio line - no, would you treat a 100V power cable to bare terminals and stripping and installing while alive, I hope not (some Americans might).

Is 100um of insulation enough to hold off occasional flights to 100V when there is no sustained arc hazard, yes,... Safe on a mains cable, not really.

Mike.

If you think about my original reply, I did say electrical separation and Class II are most appropriate.

To a degree I see why you are worried about class separation, but the whole subject is much too complex to have these blanket ideas. Sheathed cables need no reason to apply such rules, in fact we don't insist that a TV mains cable and aerial or network cable are physically separated and they are often tied together. Essentially there is no risk unless the whole lot is abused to the extent that they all short together, and even then the live mains conductors would short and pop the CPD.

It is always possible to make some scenario where there could be danger, but we need to understand that blanket rules can never stop all problems. The latest changes to the Highway Code seem to be a case in point, they will cause more danger not less, and obviously whoever did this has some kind of vendetta against vehicle road users.

POE is another idea to help manufacturers of a particular kind of product to reduce their cost without considering the system cost as a whole. It simply moves the low power "power supply" problem to someone else, who then needs more electronics and a abigger power supply, typically a router. The POE spec should match that for the data, ie every port is isolated from Earth, and any POE source should provide a class 2 output to a single port without any ground reference to at least 500V DC. Simple enough, like class 2 electric car charging, but no one is interested because they hadn't thought of that before! BS7671 is not the place to fix either problem as you probably agree. Ridiculous levels of insulation are not the solution to any problem, normal levels of 100V insulation (which almost always uses mains cable with sheath) should provide an adequate degree of safety even mixed with other services or mains cables.

David Stone said:

in fact we don't insist that a TV mains cable and aerial or network cable are physically separated and they are often tied together.

We certainly do ... in some parts of installations in buildings with LPS, as I said.

Not every installation is a "standard house".

David Stone said:

The POE spec should match that for the data, ie every port is isolated from Earth, and any POE source should provide a class 2 output to a single port without any ground reference to at least 500V DC.

Sadly, just simply saying "class II" wouldn't help the situation in a number of Part 7 installations, because a limitation of voltage is also necessary.

People inventing new systems need to consider that:

(a) Most of the world doesn't use NEC

(b) It's not all "simply dry condition" for protection against electric shock.

Good afternoon all. 

Thank you all for the replies, each of which is useful (and please do not let my reply cease the debate that has erupted).  

On defining whether 100 V audio is either Band I or Band II, I think it is debatable. 

As Mike an others point out, the RMS value (which voltage bands are tied to for AC) will vary with time, and the maximum RMS value will normally be short lived.  I would expect that higher frequencies of audio signals would be better at degrading insulation through leakage, than 50 Hz, but I also think that at those frequencies the leakage effect would not be an effective counterbalance to the shortness of the peaks in RMS voltage in terms of insulation degradation.  I have not checked that however.

As also pointed out by others, 100 V audio is likely to be isolated from earth, and so the audio voltage is not with reference to earth (i.e. it is separated).  

Hence on balance while it might be preferable to run 100 V audio entirely separately, I do not think there is a robust case to say that 100 V audio (being 100 V line to line and not with reference to earth) could not be run with Band I circuits.   

Does anyone disagree?

Thanks, once again.

Dutch of the Elm said:

Hence on balance while it might be preferable to run 100 V audio entirely separately, I do not think there is a robust case to say that 100 V audio (being 100 V line to line and not with reference to earth) could not be run with Band I circuits. 

It depends on whether those Band I circuits have separation requirements in their relevant supplementary installation standards, and those separation requirements may be for EMC or performance, as well as safety.


There's also the consideration of LPS/LPZ, and "hot" vs "cold" cabling.

And separation for safety services.

So, whilst "easy" for the general case, in practice, even not all Band I or Band II circuits can be run in the same containment as each other.

Agreed!

Yep, you don't want mains Voltages appearing on speaker cables if they run together and there is a fault, such as mice chewing through the insulation and causing a short. Or other bad situations causing mains to appear at speakers and associated equipment like amplifiers.

Z.