Re: Inspections: (was Re: mixed voltages in Altronix ALTV244 box)

["Robert Green" <ROBERT_GREEN1963@xxxxxxxxx>]

"Marc F Hult" <MFHult@xxxxxxxxxxxxxxxxxxxxx> wrote

<stuff snipped>

> IMO, what folks that do their own home automation and other low voltage
> installations in the US and Canada need to grok is that the NEC, NFPA, UL
and
> ULC address fire and electrocution prevention.

Agreed.  Even OSHA has standards:

_______________________________________________

Design Safety Standards for Electrical Systems: Special Systems -- OSHA
Standard 1910.308 1

1910.308(c)(1) -- Classification. Class 1, Class 2, or Class 3 remote
control, signaling, or power-limited circuits are characterized by their
usage and electrical power limitation which differentiates them from light
and power circuits. These circuits are classified in accordance with their
respective voltage and power limitations as summarized in paragraphs
(c)(1)(i) through (c)(1)(iii) of this section.

(i) -- Class 1 Circuits

(A) -- A Class 1 power-limited circuit is supplied from a source having a
rated output of not more than 30 volts and 1000 volt-amperes.

(B) -- A Class 1 remote control circuit or a Class 1 signaling circuit has a
voltage which does not exceed 600 volts; however, the power output of the
source need not be limited.

(ii) -- Class 2 and Class 3 Circuits

(A) -- Power for Class 2 and Class 3 circuits is limited either inherently
(in which no overcurrent protection is required) or by a combination of a
power source and overcurrent protection.

(B) -- The maximum circuit voltage is 150 volts AC or DC for a Class 2
inherently limited power source, and 100 volts AC or DC for a Class 3
inherently limited power source.

(C) -- The maximum circuit voltage is 30 volts AC and 60 volts DC for a
Class 2 power source limited by overcurrent protection, and 150 volts AC or
DC for a Class 3 power source limited by overcurrent protection.

(iii) -- The maximum circuit voltages in paragraphs (c)(1)(i) and (c)(1)(ii)
of this section apply to sinusoidal AC or continuous DC power sources, and
where wet contact occurrence is not likely.

1910.308(c)(2) -- Marking. A Class 2 or Class 3 power supply unit shall be
durably marked where plainly visible to indicate the class of supply and its
electrical rating. (See 1910.302(b)(3).)

______________________________________________

I assume these are very much like the NEC and UL requirements.  Since you
clearly are an expert in the low voltage code requirements, can you tell me
what they consider as a "Class 1 remote control circuit?"

In paragraph i-A)  they discuss a top end of 30V but in B) just below they
say "has a voltage which does not exceed 600 volts."  Does that mean the
voltage of the remotely controlled circuit?  It's not clear to me.

In your example, without the "inherent" current limiting provided by Class 2
wall-warts, wouldn't the wire have to be externally fused or have some other
method of preventing the CAT-5/battery scenario you hypothecated?

Correct me if I am wrong, but the whole point of requiring Class 2
wall-warts is that a dead short at the end of the cable will result in the
wall-wart shutting down and not burning up.

The Class 2 specification issues are well-documented in the archives of this
group and others, like:

news:sci.engr.electrical.compliance

I assume the NEC puts emphasis on insuring Class 2 use in the home because
non-compliant wall-warts have been repeatedly found to be the "point of
ignition" in housefires.

There are plenty of power supplies that have been recalled because of fire
hazard here:

http://www.recall-warnings.com/cpsc-category-adapter_power_supply_charger_ba
ttery_1.html


> They do not, and are not intended to insure dependable or
> efficient operation of electrical devices or
> to protect those devices in case of malfunction or misuse.

Agreed.  But isn't dependable operation usually a by-product of following
the code?  It's a chicken-and-egg sort of thing.  I agree, however, that
operational aspects rarely concern the inspector beyond simple function (for
instance: does a three-way switch installation actually work as expected?)

> So in the hypothetical I posed, the concern of the inspector is not
whether the
> wires would perform under normal operation, but rather what hazards would
be
> created during abnormal circumstances and how to minimize their impact.

Since it's hypothetical, we'll never know, but I would suspect in some cases
he would like to see if the device powered by the circuit actually came on.

> (Connect a length of twisted pair of CAT5 to a car battery and short the
far
> ends if you want a hint of what "low-voltage" can do.)

I had a wiring harness fire in an old Ford LTD once.  I was impressed by how
much damage it did in a very short time.  That was a long, long time ago and
I imagine harness fires are far less common  because there are a lot more
fuses, fusible links and other protections on automotive electrical systems
these days.

> So the fact that 18 AWG is much larger than is needed for the power
> requirements of any given device is quite irrelevant to the
responsibilities of
> the inspector. And so whether he understands that or not is also not
important.

It would be nice to think that he did understand, and was enforcing the
larger gauge simply because that's what the code says.  One would think the
inspector *has* to know enough about the device or the power supply to know
where the various provisions of the code come into play.  When your
inspector comes, does he ever check whether the devices function or does he
simply inspect the "cold" wiring and device markings?

> What matters is that if a device is not marked as Class 2 (as all the wall
> warts in your house _should_ be), the National Electrical Code that he is
> enforcing *requires* it to be treated as if it did not have Class 2
intrinsic
> power limitation  and so would need to be treated (at least) as Class 1
(=< 30
> volts and =< 1000VA with related higher wiring requirements) in order to
assure
> safety to people and property according to the Code.

Which means, among other things, fuses or breakers to prevent the meltdown
you posit below:

> Consider my CAT5 + car battery example:  it is clear that how much power
is
> drawn by the gizmo attached to the other end of the CAT-5 has nothing
> *whatever*  to do with how safe the combination of power source, overload
> protection and wiring is or is not.

Agreed.  As you mentioned in the beginning, the most important aspect of all
this is ensuring whatever wiring you do in your home is not a potential fire
or shock hazard.  For me, that means making sure ever power wire is fused or
otherwise protected as close to the power source as possible AND that you
use Class 2 transformers as well, even though they should have such
protection built-in.  It's so easy to fuse 12V lines with automotive fuses
that there's no reason not to overprotect everything.

Thanks for the info.  I'm about to tackle putting a skeleton video system in
the house capable of running for at least a few days from a large marine
trolling battery.  The problem is that DC voltage drops off pretty quickly
so I need to locate the power supply close to the point-of-use.  I may end
up using two batteries just to avoid all the issues involved in transmitting
DC power over a distance.

One of my CCTV transceivers that supplies power, audio and video over a
single RG6 cable has a variable voltage AC to DC power supply that you can
adjust to deliver 12V at the camera, but without DC-DC converters, there's
no way to crank up a 12V battery's output quite so easily to account for the
voltage drop over long distances.

--
Bobby G.