Statpower failure

[hugh fellows]

Hi Arch.

When you said in your recent post that you had your unit connected to
220 V at the time of failure ,it set off a little buzzer in my brain. I
had almost an identical situation occur in some mobile equipment that my
company manufactured. Interestingly enough the scenario involved a
battery charger and an air conditioner running off a ship to shore type
power connection. Since that time I have observed this phenomena on
several occasions. It is often referred to by electricians as a "lost
neutral" condition. Somewhere in the system (usually at the ship to
shore plug) the neutral suffers a serious increase in resistance or a
complete disconnection. When this happens the appliance of greatest
impedance (charger) can have it's connection to the neutral effectively
replaced by a connection to the other 220V wire thru an appliance of
much less impedance than itself (air conditioner). The end result is
that the charger ,now operating off twice it's design voltage ,soon
gives up the ghost. I lost two of these chargers in 1980 before I saw
what was going on. The culprit was a bad neutral lug on the ship to
shore plug. The fault could have been anywhere along that neutral
conducter. If you are connected to 220V and your 110V loads are divided
to equally load the two hot conductors you must be careful to maintain
that neutral lug in good shape or this phenomena can kill a lot of good
equipment. Possibly this is the same situation that got you statpower.
Good luck in tracing down your problem . This is one possibility to
consider.

Regards
Hugh Fellows
77 Eleganza
in War Eagle Country

 

[hugh fellows]

Hi Arch,
Sorry to take so long to reply but I'm on the digest and gone from
terminal all weekend.
I hope I can explain this to you in such a way that it can be
understood. I understand a lot of things that I can't explain . Let's
try this. On a blank sheet of paper, draw a solid vertical line from top
to bottom approximately 2" from left hand side of sheet. Four more
inches to the right draw another. Directly between these 2 lines draw
another ,but make it a dashed line. These represent the wires for a
110/220V power system. Now draw a load resistor etc(the charger)
connected between the left wire and the neutral wire(center dashed
line). In line with this load between the right wire and the dashed
neutral, draw another resistor to represent an air conditioner load.
There are usually several loads on each side of the neutral in most such
systems, but not all will be active(on) at one time. Usually the wiring
will be done in such a way that the current sum on the left hand loads
approximately equals the current sum for the loads on the right. This is
called a balanced condition. If you think of each of these wires as a
common connection point (a node), then the sum of all current flowing
into a node must always equal the sum of all flowing out. Suppose only
these two devices are on(for simplicity) .
since the left and right wires at any instant of time are opposite in
polarity and assuming conventional current, the current will flow from
the + to the - . If both loads carry exactly the same current you
can see that our nodal balance requires that the current in or out thru
the neutral wire must be exactly zero. In this balanced state we could
remove the neutral wire and everything would continue to function
normally. But we know our charger will not draw as much current as our
air conditioner, hence by our nodal analysis, the neutral wire must now
carry the imbalance. In our case this imbalance is substantial. So if
for some reason we developed a disconnect at the neutral spade(or
anywhere else) the system would try to force all the current normally
flowing thru the air conditioner thru the charger. This is over
simplified but You get the picture. The current going thru the charger
is a lot more than it should be. For a more accurate picture, think of
the two loads as now being connected in series and since the impedance
of the charger is several times the impedance of the air conditioner,
essentially all the voltage drop will occur across the charger, causing
it to operate at close to twice its rated voltage. I hope You are able
to piece this together. It's about the best I can do at explaining. This
may not be your problem but it occurs from time to time in 110/220V ship
to shore type connections. When it happens it often kills some appliance
etc. . So, if you are going to operate on a 110/220V system , keep
that neutral spade and conductor in good shape or else!

Good luck,
Hugh

 

[hugh fellows]

Good Morning Arch,

You are correct when you say we have no 220V loads. All our appliances
are 110V, but you need have no 220V appliances for this to happen. It
has nothing to do with 220V loads, only 110V loads operating on a
110/220V system. From the diagram we drew yesterday we had 2 ea. 220V
busses and a single neutral buss in the center. Erase all loads but one
on each side of the neutral buss. Now imagine that somehow the wire
coming to the neutral buss is disconnected. We know that there is 220V
potential difference between the two outside busses. If you now look at
the system with the neutral removed, you will see that you now have two
110V rated appliances connected in series across the two 220V busses.
If both appliances were exactly the same impedance, things would even
continue to operate on these 110V appliances and nothing would be
damaged. However, when one of these loads is a battery charger (High
impedance) and the other is an air conditioner ( low impedance) the
voltage drop across each appliance will no longer be equal to 110V. The
voltage drop across the two loads in series will still be 220V but MOST
OF THE 220V DROP WILL BE ACROSS THE AIR CONDITIONER (maybe 190V). This
is going to give the transformer and semiconductors of the charger a lot
of undue stress. As you can see you don't need any 220V loads for this
phenomena to occur, you produce a 220V load out of two 110V loads which
are essentially connected in series across the 220V lines. I hope this
makes sense. If not maybe someone else can help me explain.

Have a GREAT day Arch.

Hugh

 

[travis martin]

> Arch PS would a GFI see this problem and stop it?

No, but an MOV (metallic oxide varistor) as found in a surge protector
would. You might consider using a surge protector; they are dime-a-dozen at
computer stores. An MOV is extremely fast-acting and sensitive to
overvoltage. If it is a short duration noise spike, it simply shunts it and
no harm, no foul. On an overvoltage condition like Hugh is explaining, it
would still attempt to shunt the overvoltage but, since it is of long
duration, would likely sacrifice itself doing it. This is still far better
than the Stapower sacrificing itself. The little surge protector gets a
hero's funeral and the Statpower soldiers on. I'd recommend it.

Best regards,

Travis

 

[travis martin]

>I will admit my first thought is-----if I have lost the
> ground what is the surge protector going to do with the voltage. Oh
> well-----going to go out and see if the ground and neutral are tied
> together in the GMC breaker box.
>
> Take Care
> Arch
>

No, actually what happens is that it shunts the overvoltage to the neutral
connection at the outlet to which it's connected. In Hugh's hypothetical
example, this neutral is actually connected to the opposite phase 120 volt
line through the A/C.

The point is, though, that the Statpower never sees more than about 140
volts before the MOV clamps it. And it only sees the 140volts for a very
brief time--microseconds--not long enough to overheat anything.

Think of the MOV as the electrical equivalent of the pop-off valve on your
air compressor. If the controls hang and pressure continues to build, at
some point the pop-off lets go and bleeds off the excess. This protects the
tank and lines from the possibility of rupture from excess pressure. The
difference is that the MOV usually dies in the act if the surge is of more
than a few microseconds long. They turn black and stinky, but you have a
positive indication that there was a problem and that it caught it.

Incidentally, a lot of modern equipment has MOV's built in to the design,
usually just downstream of the line fuse so that if an overvoltage occurs it
blows the fuse before the equipment is damaged. I don't have any idea
whether Statpower included this in their design or not; perhaps Henry could
find out as part of his quest. If this is the case, the computer store surge
protector is redundant.

Travis

 

[hugh fellows]

Hi Arch,
Sounds like you understand how this works now. You were correct in
measuring the voltage at the charger receptacle or connection. However
you should do this with the loads on, I.E. don't just measure the
voltage of the unused outlet. Try to replicate the situation as it
existed at time of failure. Turn on the charger (pulling juice from one
of the 220V hot wires), then turn on the air conditioner. Be sure to
use the air conditioner which runs off the OTHER HOT WIRE . Now measure
the voltage at the charger plug.
If you still see from 110 to 125V , then probably this is not your
problem. However you might still try wiggling wires and the ship to
shore plug while someone observes the voltmeter for needle jump.
Sometimes these are intermittent problems which are not total
disconnects of the neutral, but just a poor quality connections .

Even if this turns out to be unrelated to your problem, at least you now
have reason to pay attention to the condition of your plug when you
connect it to 220.

Happy Trails
Hugh Fellows

 

[hugh fellows]

Hi Arch,

Sorry, I forgot to answer all your questions. " Would a GFI receptacle
protect us against this problem"? I feel pretty sure it wouldn't!
Here's why. These devices measure the current going out from and
returning to the receptacle. If they are the same then no current is
leaking to ground through some poor person. In our case, although the
currents may not be what they should be, the current going out should
still be same as current coming back to the GFI. You could give some
protection to the particular device by equipping the device attachment
wire with it's own inline fuse which is sized approximately 20% over the
full load rating of the particular device. However other devices are
still at risk. Just keep your power cord and plug in good shape and
securely plugged in especially when operating from 220V. I always bungee
cord mine to the outlet box to avoid problems caused by people bumping
the cord.

Happy Trails
Hugh