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'[EE] Element corrosion'
2009\08\06@040543 by Jinx

face picon face
The element in our hot water cylinder failed yesterday (didn't quite
make its 19th birthday) and an electrician came today to replace it

Looked OK on the outside. Clean, shiny. But, oh, the horrors on
the inside. A little corroded. Just a little

http://home.clear.net.nz/pages/joecolquitt/element1.jpg

http://home.clear.net.nz/pages/joecolquitt/element2.jpg

Looks like a nasty surgery thing I saw on Discovery

This must be long-standing corrosion and it absolutely amazes me
that it has taken until now to pop the circuit breaker. Surely the
winding must have been wet before yesterday. After it dried, most
of the filler just fell out. My nephew up north is doing an electrician's
apprenticeship and says he sees that sort of breakdown regularly

So I was chatting with the sparkie, as you do, and asked him why
the damage was closer to the Active/Live connection. (you can see
in the first picture that the damage pretty much stops where the
element turns around, ie halfway between A and N). He couldn't
say. He said that he asked once at college why, if the current is
AC, that Active and Neutral aren't regarded as interchangeable.
But the answer he got was "they just aren't"

Anyway, I've wondered about it too. I've thrown out IEC leads,
like on the kettle for example, because the Active connection has
suffered heat and corrosion damage. I'd have thought that elements
are more or less just resistive, nothing complicated wrt power for
example

Why does the corrosion happen that way ?

2009\08\06@044509 by Marcel Birthelmer

picon face
>
> So I was chatting with the sparkie, as you do, and asked him why
> the damage was closer to the Active/Live connection. (you can see
> in the first picture that the damage pretty much stops where the
> element turns around, ie halfway between A and N). He couldn't
> say. He said that he asked once at college why, if the current is
> AC, that Active and Neutral aren't regarded as interchangeable.
> But the answer he got was "they just aren't"

Would it be similar to why telegraph signals (or was it something
else?) use 0V and -48V, rather than 48V and 0V - that is, because
"everything else" (air etc.) are at "true ground" = 0V? So in this
case, all the water would be more or less at ground potential
(basically neutral) while all the corrosion happens where there is a
potential difference, e.g. the "hot" side.
Though that doesn't explain why this happens at AC... you'd think any
corrosive effects in one half-period would undo themselves in the
other half-period, the way that battery terminals do...

Ah well, just a shot in the dark.

2009\08\06@053935 by Brent Brown

picon face
On 6 Aug 2009 at 20:04, Jinx wrote:

{Quote hidden}

Hmm, I haven't thought this through too far but... Remember that in NZ neutral and
earth are bonded together at the switchboard. Your cylinder is earthed, and water
conducts. So the potential difference between Phase and water (230V) is greater
than Neutral and water (0V). That doesn't explain the mechanisim of corrosion, but
it might be a clue as to why it's worse at one end than the other?

Without an earth leakage breaker your dodgy element was probably quite happy  
heating the water... the usual amount by power in the element, plus a bit extra
through conduction!

PS. My hot water cylinder is leaking so needs replacing very soon.. I'll check out the
element when it comes out.

--
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  spam_OUTbrent.brownTakeThisOuTspamclear.net.nz


2009\08\06@083419 by Alan B. Pearce

face picon face
>Why does the corrosion happen that way ?

Probably because the neutral side is effectively at earth potential in an
MEN system, so you don't get the potential difference between the neutral
end and the earthed outer casing.

Same thing would happen with the IEC connectors.

2009\08\06@095257 by Spehro Pefhany

picon face
At 04:04 AM 06/08/2009, you wrote:
{Quote hidden}

If the insulation is damaged, the 'hot' wire will conduct electricity
to the ground on your hot water heater. It will bubble like crazy,
corrode electrolytically and so on. The neutral will be almost at
ground potential, so no such activity takes place. The element (while
it is intact) acts like a voltage divider (with some taps conducting
current off to ground through the water).

This is a very graphic illustration of why a solid ground is essential
for safety!

The current flowing from the exposed element might be enough to be fatal
if the ground conductor did not safely carry it away and it instead found
a path to ground through a human chest.

Can I presume that a GF(C)I (or whatever you call them there-- Residual
Current Device?) is not fitted to the water heater? I imagine it would be
difficult to control leakage under normal conditions with a water heater.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2009\08\06@113832 by Herbert Graf

picon face
On Thu, 2009-08-06 at 20:04 +1200, Jinx wrote:
> So I was chatting with the sparkie, as you do, and asked him why
> the damage was closer to the Active/Live connection. (you can see
> in the first picture that the damage pretty much stops where the
> element turns around, ie halfway between A and N). He couldn't
> say. He said that he asked once at college why, if the current is
> AC, that Active and Neutral aren't regarded as interchangeable.
> But the answer he got was "they just aren't"

In NA the neutral is bonded to ground at the panel. The water in the
water tank is at the same potential (the ground in the panel is a wire
connecting to the cold water pipe). So, at the water heater, the
potential different between the neutral lead and the water is very small
(perhaps up to a volt or two due to a voltage drop across the neutral
run to the panel). Obviously the potential different between the hot and
ground is basically the full amount.

TTYL

2009\08\06@122821 by Marcel Duchamp

picon face
Jinx wrote:
> The element in our hot water cylinder failed yesterday (didn't quite
> make its 19th birthday) and an electrician came today to replace it
>
>
> This must be long-standing corrosion and it absolutely amazes me
> that it has taken until now to pop the circuit breaker. Surely the
> winding must have been wet before yesterday.

Maybe not.  My limited understanding is that water heaters use a
sacrificial anode that has a lower potential than the rest of the
heater.  It slowly corrodes away during the life of the system.  When
it's gone, corrosion continues with whatever material has the next
higher potential.  This rapidly destroys the heater at that point.

Water heaters are sold with particular lifetimes - eg, 15 year, 20 year,
etc.  Same heaters, different longevity.  How?  Longer sacrificial
anode.  An electrician told me to buy the shorter lifetime heater at a
lower price and replace the anode with a 30 year one.

2009\08\06@124336 by Carl Denk

flavicon
face
The ground and neutral being at the same potential is theoretical, but
practice due to either errors or damage to the wiring, this might not be
true. The ground and neutral (In the USA) are required to be bonded in
the main panel legally, and that point should be grounded with a ground
rod, or possibly a copper buried pipe. But, with the possible presence  
of plastic piping and accessories, like water meters, the ground paths
may get destroyed, or even never in existence. Also when there is heavy
current (for wire size) flowing in the neutral, ther will be a slight
voltage drop due to the resistance of the wire and terminations. There
is a requirement to install a heavy (#4 or #8 I believe) copper ground
wire to the piping at both sides of the water meter, but with plastic
underground piping that is futile, and a ground rod must be used.  When
I installed our standby generator, I was careful to bond all grounds and
neutrals starting at one panel point. The service disconnect is not in
the main panel which has a main switch for the main breaker panel.
Herbert Graf wrote:
{Quote hidden}

2009\08\06@133037 by Dr Skip

picon face
That's for galvanic corrosion, and when copper and iron are both present, the
iron corrodes first. With a magnesium or aluminum rod, it will corrode instead.

Chances are, it's the heat cycling that fatigues the copper over time to let in
a bit of water. Any manufacturing defect or inconsistency would facilitate that
too. Then steam generation inside the tube via the crack, and the cycling,
erode further and 'pump' water in and out. Once water is in contact with the
actual wire, then it's effects come into it. Note that there will be a
potential gradient along the wire, beginning, and highest, with the hot
connection. It should cause the worst looking damage there, and it appears to
be so in the pic.

Sacrificial anode replacement will protect the tank lining, while electric
elements will just need to be replaced periodically. A GFCI breaker would alert
you to the element breach. Replacing the anode on a regular schedule will
protect the tank.



Marcel Duchamp wrote:
{Quote hidden}

2009\08\06@202051 by Jinx

face picon face
> Can I presume that a GF(C)I (or whatever you call them there--
> Residual Current Device?) is not fitted to the water heater?

Only a resettable 16A circuit breaker in the fuse box

> If the insulation is damaged, the 'hot' wire will conduct electricity
> to the ground on your hot water heater. It will bubble like crazy

The electrician did ask if power bills had been unusually high lately

So any defect in the insulation will make a hot spot that spreads
as metal is eaten

Eventually, I suppose then, enough of the wire conducts to trip the
circuit breaker. AIUI water is a poor conductor compared with
wire. So if the portion of the element closest to the active terminal
(where the potential difference is greatest and where the corrosion
starts) was more electrically insulated from the water this might delay
corrosion

Also, for a few days before the element finally opened, I'd noticed
what I thought were air locks in the hot water pipe. The HW taps
sputtered before good flow. I presume that this was due to the
bubbling you mention. If this is gaseous steam inside the tank it
might be a pressure or condensation effect at the taps


Thanks for all comments/explanations

2009\08\06@214756 by Richard Prosser

picon face
Hmm.
I've been getting high power bills over the last 2 months. I wonder if
I'm getting a similar problem (cylinder is ~20years old). Could just
be the cold weather.

I'm guessing the "airblocks" were a mixture of hydrogen & oxygen from
the water breakdown. If I get the same, maybe I'll try applying a
match! Or maybe not - might blow my taps off.

RP

2009/8/7 Jinx <joecolquittspamKILLspamclear.net.nz>:
{Quote hidden}

> -

2009\08\07@000902 by John Gardner

picon face
> Or maybe not - might blow my taps off.

Or worse...

2009\08\07@002327 by Sean Breheny

face picon face
I don't think that 60Hz AC will hydrolyse water very much. That's
pretty much a DC effect. I would suspect steam first.

Sean


On Thu, Aug 6, 2009 at 9:47 PM, Richard Prosser<.....rhprosserKILLspamspam.....gmail.com> wrote:
{Quote hidden}

2009\08\07@003307 by Don Kovalchik - W8DPK

flavicon
face
Jinx wrote:
> The electrician did ask if power bills had been unusually high lately
>  
If any current was leaking through the water, it would cause the water
to heat up with the same efficiency as the heating element.  A watt is a
watt, no matter if it is from the heating element or the water
resistance. Once the water is hot enough to satisfy the thermostat, the
thermostat switch will open and the current will stop. No unusual power
bills will result.

I suspect that the conductance of the water is much less than the
conductance of the heating element.  My water heater element is rated at
5000W at 240V, which is about 21A, and is fused (circuit breaker) at
25A.  If the water was very conductive, the extra current should trip
the breaker.

--Don--


2009\08\07@050647 by Jinx

face picon face
> If any current was leaking through the water, it would cause the
> water to heat up with the same efficiency as the heating element

That's what I thought when he asked. But he's the expert. If the
element is 2000W, and only a part of the power bill, there'd
have to be a lot of leakage to push the power bill up noticeably

Although there really is a lot of corrosion muck around the base
of the element, which I expect are non-conducting salts, so any
leakage would still have to go through the water / salt ions. He's
replaced the copper-sheathed element with a stainless steel one,
which he reckons will last longer

2009\08\07@085925 by Carl Denk

flavicon
face
I couple of years ago, we got a high (x2)  electric power bill. A search
found a small under desk resistance 120 volt , maybe 1000 watts heater,
the thermostat stuck "on". Assuming your power meter has a rotating
visual disc, with everything you can shut off easily, and no motors,
heaters, etc. "on", the disc should revolving slowly, say around less
than 1 revolution per second, not 10 per second.

Richard Prosser wrote:
{Quote hidden}

>> --

2009\08\07@095424 by Chris Smolinski

flavicon
face
Well, this was an appropriate thread - the element on my heater just
failed this morning. Bottom one, of course. Draining the tank now,
I'll post pictures if it looks like a spectacular failure.

--

---
Chris Smolinski
Black Cat Systems
http://www.blackcatsystems.com

2009\08\07@192603 by Jinx

face picon face
My nephew the apprentice sent me this in our correspondence about
heater elements

===================

OK, here's the scenario.  I get called to a house with no hot water and
their hot water circuit breaker (16A with the circuit cable size being
1.5mm 2-core plus earth) would trip a minute or two after being reset

So, I took the cover off the cylinder and took a look at the element,
thermostat etc. reset the breaker and tested to make sure I had power
out of the thermostat into the element. test showed I had 230v on the
Phase side of the element, so I put my clamp meter over the Neutral
coming off the element, expecting to have roughly 8 amps due to it
being a 2kW element, but that wasn't the case, I had only a few mA,
if that. I jumped to the conclusion that it was a faulty element as I had
just traced the problem back to it

I drained the cylinder, replaced the element, refilled the cylinder and
wired in the new element.......... only to have the exact same thing
happen!! A little anoyed, and puzzled, I decided to put my voltage
testers on the incoming to see if there were any abnormalities. On
the isolating switch, I tested between phase and earth - 230v, phase
and neutral - 230v, and then...........earth and neutral - 230v dun dun
dunnnnn. I was on to something, as there should be no potential
between earth and neutral as they are connected back at the Earth-
Neutral bars in the switchboard

This also explained why the element wasn't drawing any current.so,
there must be a break in the neutral somewhere. But where?  I
disconnected all the incoming cables at the cylinder and disconected
the hot water circuit's phase, earth, and neutral. I tested each cable
with my multimeter and found there to be a dead short between
phase and neutral at this stage

This told me that the fault was somewhere on the supply cable to the
cylinder. The most likely place to me would be the ceiling, so I jumped
up there with my torch and had a look around, working my way from
the top of the switchboard, towards  the hot water cupboard and
couldn't see any breaks or niks in the cable, but i did notice a bit of
blackness around the hole where the wire went down the wall, so I
pulled the slack up the wall and found the wire to have a big chunk
out of it with some melted copper and mice bite marks on the insulation.

So, I pretty much changed the element for no reason.

=============

I'm pleased that he found the problem (before there was a house
fire) and is applying some logic, but see he missed one early step
that would have saved a lot of time

2009\08\08@012159 by Brent Brown

picon face
On 8 Aug 2009 at 11:20, Jinx wrote:

{Quote hidden}

Yeah, a double check with an ohmmeter (and also a saftey check with an insulation
tester) after it was removed would have told him the element was ok, or if he'd
measured phase to neutral voltage at the element. Sometimes the tests you do are
limited by the options available...  I imagine that the neutral junction wasn't easily
accessible like the two brass screws on the back of the thermostat, probably used
earth as reference instead for testing voltage - clamp on ammeter test was useful.
Sounds like he made another mistake later on when he first replaced the
thermostat... said he measured phase to neutral 230V, which doesn't seem possible
when the fault was found to be a short between phase and neutral. However, this
could be explained by the fault progressing as he did say initially it appeared to take
a period of time to trip the breaker.

--
Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brownspamspam_OUTclear.net.nz


2009\08\08@042933 by Richard Prosser

picon face

2009/8/8 Carl Denk <@spam@cdenkKILLspamspamwindstream.net>:
> I couple of years ago, we got a high (x2)  electric power bill. A search
> found a small under desk resistance 120 volt , maybe 1000 watts heater,
> the thermostat stuck "on". Assuming your power meter has a rotating
> visual disc, with everything you can shut off easily, and no motors,
> heaters, etc. "on", the disc should revolving slowly, say around less
> than 1 revolution per second, not 10 per second.
>

Thanks Carl,

The mechanical meters got replaced with electronic ones a year or so
ago, but there is a flashing light to give an indication of activity.

Anyway, I reckon I should be able to measure any leakage current by
either looking at the difference between the neutral and phase
currents with a clamp meter, or by disconnecting the neutral and
measuring the phase current alone.
But itś most likely to be the colder weather as we´ŕe now in the
middle of winter & have the heater on a fair bit.
Power usage has always been higher than I expect though so it warrants
a bit more investigation. I a bit worried that one of the hot water
taps seems to work in  an almost ïnstant hot water mode which suggests
a hot water leak somewhere I  can´t  see or get at. - eg inside the
concrete slab. But then itÅ› the tap nearest the cylinder.

RP

2009\08\08@083458 by Carl Denk

flavicon
face
In the USA, the ground wire could be legally, and is commonly a size or 2  
smaller than the main conductors including the common. With the main  
conductors pulling their maximum legal amperage, (no common, but  
possible), a break in the common and short to the ground could overload  
the ground wire, heat it, and caise a fire. :(


On Sat, 08 Aug 2009 01:21:05 -0400, Brent Brown <KILLspambrent.brownKILLspamspamclear.net.nz>  
wrote:

{Quote hidden}

2009\08\08@125735 by M. Adam Davis

face picon face
On Sat, Aug 8, 2009 at 8:34 AM, Carl Denk<RemoveMEcdenkTakeThisOuTspamwindstream.net> wrote:
> In the USA, the ground wire could be legally, and is commonly a size or 2
> smaller than the main conductors including the common. With the main
> conductors pulling their maximum legal amperage, (no common, but
> possible), a break in the common and short to the ground could overload
> the ground wire, heat it, and caise a fire. :(

True, except that the wire has a very large margin built into it.  12
gauge wire can twice what it's rated for before it becomes a problem
in most houses, so the circuit breaker should still cut the flow
before the ground wire becomes very warm.

However, heat buildup in a well-insulated portion of the wall may
still cause a fire even if the wire is only acting as a 1watt heating
element.

Anybody interested in testing heat buildup should put a 1 watt
resistor running a full wat inside an insulated box (disposable foam
cooler) and monitor the interior temperature over a day or three.

-Adam

2009\08\09@023528 by Sean Breheny

face picon face
On Sat, Aug 8, 2009 at 12:57 PM, M. Adam Davis<spamBeGonestienmanspamBeGonespamgmail.com> wrote:
> Anybody interested in testing heat buildup should put a 1 watt
> resistor running a full wat inside an insulated box (disposable foam
> cooler) and monitor the interior temperature over a day or three.
>

I have done that (in the process of trying to make a temperature
controlled oven) and it melted its own solder connections :)

Sean


> -Adam
> -

2009\08\10@111334 by Michael Rigby-Jones

flavicon
face


> -----Original Message-----
> From: TakeThisOuTpiclist-bouncesEraseMEspamspam_OUTmit.edu [RemoveMEpiclist-bouncesspamTakeThisOuTmit.edu] On
Behalf
{Quote hidden}

and
> wired in the new element.......... only to have the exact same thing
> happen!! A little anoyed, and puzzled, I decided to put my voltage
> testers on the incoming to see if there were any abnormalities. On
> the isolating switch, I tested between phase and earth - 230v, phase
> and neutral - 230v, and then...........earth and neutral - 230v dun
dun
> dunnnnn. I was on to something, as there should be no potential
> between earth and neutral as they are connected back at the Earth-
> Neutral bars in the switchboard

Given a single phase system, I don't quite understand how there could be
230v between Live and Neutral at the same time as 230v between Neutral
and Earth?

Mike

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2009\08\10@134936 by Isaac Marino Bavaresco

flavicon
face
Michael Rigby-Jones escreveu:
{Quote hidden}

I have seen places where the mains are at 127V and they connect showers
and heaters on a phase-phase circuit. So, where you should expect to
find Phase-Neutral-Ground you have indeed Phase-Phase-Ground.

This is not my area of expertise, but I know there are several possible
installation configurations (Y, delta, split-phase, etc.) Perhaps one of
them may explain this odd voltage measurements.

For instance, in a three-phase system without a fourth wire for neutral,
if you ground one of the phases (making the neutral of it), then you
could get these same results if using the two remaining phases to power
the heater.

It is probably out of the standards and possibly illegal in some places,
but in may father's farm the power grid was 3-phase 127V without neutral
and we used phase-phase to get 220V for the houses. The showers didn't
even had ground connections and never anybody got an electrical shock
while bathing.

Regards,

Isaac

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