On Fri, 24 Jun 2011 11:07:08 +1000, Sylvia Else
wrote:

On 24/06/2011 8:53 AM, wrote:
On Thu, 23 Jun 2011 12:27:06 +1000, Sylvia Else
wrote:

On 21/06/2011 12:11 PM, Jeffrey Angus wrote:
On 6/20/2011 7:16 PM, William Sommerwerck wrote:
Remember, it ONLY takes ONE switch to OPEN a series circuit.
That is ALL that is required to control whether or not the
element gets hot or cools off.

You were not paying attention! Any heating device that uses both "phases"
requres TWO switches to open it.


Oh, and by the way, since you brought it up, the bit about
both of the 240 volt sources being "hot with respect to
neutral". The heating elements are connected ACROSS the 240
VAC source, NOT split with each half going from the two
sources to neutral. (That would require a double contact to
turn each PAIR of heating elements on and off.)

I'm not sure what you're talking about. First, there is no "240V"
source in
my condo. There are multiple 120V sources from which you can get higher
voltages by spanning them. (I assume each voltage is referenced to some
"neutral" point.) In my preceding apartment, I took advantage of this to
build a break-out box -- all to code, I have several electrician
friends who
advised me -- to provide individual lines for my class A power amps.

As someone else kindly pointed out, this oven has one side of its
elements
hard-wired to AC. Bad, bad, bad, bad idea.

Damn you are ****ing dense William.

The heating element from one hot lead to the other hot lead.
At NO time is there a connection to Neutral. Hence it ONLY
requires a SINGLE pole switch to open a SERIES circuit.

I see no inconsistency. A correctly functioning heating element can be
turned off, in the sense of no longer heating, by means of a single pole
switch.

But what about a non-correctly functioning heating element? One, for
example, that's shorted to its grounded covering somewhere along its
length. This could result in part of the element overheating, and an
owner noticing this would likely respond by turning the appliance off at
its switch. In the case of a heating element that spans two 110 v lines
in opposite phase, turning it off by a single pole switch won't have the
desired effect. Better hope that an earth leakage detector is fitted in
the supply lines, and works.

Sylvia.
I had exactly this happen over a decade ago. The center resistance
element started arcing to the outer sheath. This action was clearly
visible. When turning off the oven didn't help, I did what any
intelligent person should do - I turned off the circuit breaker.


I'm not so convinced that people who aren't used to fiddling with the
breaker box would immediately think of that in the heat (?) of the
moment. IMHO the switch on an appliance should disconnect the power, at
least if the appliance and supply are correctly wired. If the appliance
uses live and neutral, then a single pole switch is sufficient, but if
it uses two live wires, it should have a double pole switch.

Leaving aside the fault that occurs while an appliance is in use, if
parts of the appliance are live when the appliance is turned off, then
it can still go up in flames if a fault develops within. This was not
such an infrequenty occurence in the UK when TVs weren't earthed and had
unpolarised power cords which could result in the chassis (!) being
live, even when the TV was turned off.

Sylvia.

Let's take a look at what will happen if the heating element fails by
shorting from the resistance element to the sheath. I will assume
that the failure I observed is worst case - an actual arc formed.
This gradually (1-2" per minute) moved toward the end of the heating
element. As the length of the resistance element got shorter, the
current would rise. Eventually the circuit breaker would trip. The
chance of a fire was very small. Most modern kitchen ranges have self
cleaning ovens which clean by heating themselves up high enough the
grease and spills are oxidized.

As I stated before, the problem was not in the design. Granted,
William Sommerwerk would not have received a shock if both lines were
interuurpted. However, that's not absolute protection. Switch and
relay contacts do weld together, triacs do short. One particular
brand of circuit breaker has been banned because the contacts tend to
weld together.

The chain of events is simple - The range was designed to meet certain
standards. William Sommerwerk assumed it was designed to his
standards. What is more, he was so confident in his assumption, he
failed to take the most emementary precaution of actually testing that
the terminal was not live.

Perhaps some may feel I am paranoid, but I have observed enough cases
where things were not as they should have been that I don't trust
labels, nor do I trust circuit breakers. Heck, I don't even trust a
simple neon bulb tester unless I check it!!

I've seen licensed electricians swap live and neutral wires in an
outlet box. I've seen a breaker box where the position labeled
'range' actually held two 15 amp breakers. I've seen a 12 gauge
(rated for 20 Amps) wire hooked to a 45 amp circuit breaker. I've
seen 'TN' wires (for installation in a conduit) strung across an attic
to power a ceiling fan in the room below.

PlainBill