Voltage Drop Calculations with Amprobe INSP-3 Tester

[glene77is]

101210-2127 EST

glene77is:

Replace the points and the capacitor across the points.

A good capacitor is very important. Also if you can find points for the VW with a hole in the middle of one contact
I believe you will get longer point life. This was an invention of Ford in the early 50s. I don't think it was the late 40s.

Mechanical contacts in a DC circuit show a migration of contact material in one direction producing a mound on one contact and a cavity in the other. The hole in the contact where the mound grows allows some of the metal to blow thru the hole.

The importance of the capacitor is that it reduces the rate of rise of voltage when the points open. It is also part of the compound resonant circuit of the primary and secondaries. Basically there are two tuned circuits with mutual coupling.

.

Gar,
I remember my dad using points with a hole in one contact.
As I understand it, you are right on all points (pun intended).


Temp is 15 F here this AM, and a day long inside job.
In the VW transporter engine, I run 50% Lucus with the 20-50 oil.
May have to place the 250 W incandescent light bulb
under the oil pan in order to warm up the crank case.
Sometimes also hook up a charger and shoot ether.
Have two batteries mounted in an aluminum ammo case,
inside the Van cab, in the back.
Helps on starting cold, a trick I figured out in New York.

My first car was a 57 Beetle, in Nyack, New York.
Had 6V system, in which I mounted a pair of 6V batteries,
on switches so they could be flipped into series for 12V.
Used the 12V for some accessory tool.
Mounted Volt and Amp meters on the dash, along with the switches.
Reverse dished the rear wheels, and though I was cool.

Have a better DMM on order, which will help with my tinkering.
Still use the analog clamp-on lots.
Your experiment is on my schedule.
Thanks.

 

[glene77is]

Gar,
I was rolling this over in my mind's eye,
and had to point my finger at several points to resolve.
The scenario is this, testing for hot/neutral/egc impedances
with reference to the "Amprobe Insp-3" test method.

=gar;1265374]101206-2015 EST

My guess is there is a problem on the neutral circuit. If the wire size is the same on both hot and neutral then as noted by bob the voltage drop should be about the same.

>>> In older Residentials, the EGC was smaller gauge,
and was allowed to be twist-wrapped in the 1960's.
I figure that 120 Volts and 5 Ohms will pass 25 Amps,
which is enough to trip a 20 Amp OCPD (in 30 seconds aprox).
So, 25 Ohms resistance, or 1 Ohm as pointed out in one of my handbooks, could work OK, but will throw the test readings off.
The old EGC can be twisted,
but the old Neutral will be under a gas-tight screw / wirenut contact .
Different resistances, thus different readings.

Keep in mine with the measurements I described above the reference point is the main panel neutral bus as connected to the EGC bus in the main panel.

>>> Again, Neutral vs EGC impedance differences
will alter the readings.

I just do not expect to see a higher voltage drop on the branch circuit neutral vs its hot wire.

>>> Again, Neutral vs EGC impedance differences
will alter the readings.

. . .

Install an outlet at your main panel directly connected to the neutral bus and EGC bus, and the hot thru its own breaker.

>>> For a developmental test, this is a superb method.
You are (silently) pointing to the fact that
"Neutral vs EGC impedance differences will alter the readings".

. . . Run a long extension cord from this main panel outlet to the location you want to test.

>>> For a developmental test, this is a superb method.
Again, Neutral vs EGC impedance differences will alter the readings.

For #12 copper wire the loop resistance per 1000 ft
is about 1.6 ohms.

>>> The problem of "Neutral vs EGC impedance".

A 12 A load would produce a change of voltage of about 1.92 V, or about 0.91 V on the neutral and maybe a little more on the hot wire.

>>> The problem of "Neutral vs EGC impedance".

At my work bench I get a change of 5.2 V from 134.2 to 119.0 from a 1500 W heater. There are various wire sizes, and 5 circuit breakers in the circuit, and I believe 2 plugs. The neutral path voltage drop is 1.5 V, and that makes the hot path about 3.7 V.

>>> The problem of "Neutral vs EGC impedance".

I just used the EGC at the bench for my probe back to the main panel.

>>> (Silently) covering the problem of "Neutral vs EGC impedance".

My repeated comment

The problem of "Neutral vs EGC impedance"

leads me to think that an instrument which is making these readings
will produce a reading which includes 'The problem of "Neutral vs EGC impedance"'
and the instrument will not be able to resolve the issue.

In summary:
(1) the instrument's Voltage Drop reading
will be accurate enough,
(2) but the values calculated on the basis of Neutral & EGC impedance
being the same will not be accurate,
and can only be used to indicate
that there is a difference in Neutral & EGC impedance.
(3) Perhaps that problem (referring to (2))
is what the instrument is intended to indicate.

My conclusion about an instrument
that gives Neutral vs EGC impedance readings :
The instrument does not indicate exact values for direct usage,
but rather,
the fact that there is a difference in impedance
which needs to be checked out as a problem in the circuit under test.
(maybe a loose neutral connection, or a connection between the neutral and egc, or a loose egc connection.)

I wonder what a loaded EGC would look like ?
I wonder what a loaded EGC would look like ?
Perhaps the Neutral impedance would be much higher than the EGC's,
because of a voltage being impressed on the EGC line.
(Ex, where a load is passed back on the EGC, because the neutral at some switch is used for a switch-loop to feed a switch-leg,
and the guy needs a return wire. )
(Ex. where some Dimmer or indicator light on a switch
is using the EGC for a return, as was common practice. )

This is a good exercise.
Might purchase it, having a better understanding of what it can do.