This is an interesting discussion of a topic that recurs frequently. I
agree with Jim K9YC that house wiring should preferably bond the Neutral
circuit to Ground at the service entrance, and nowhere else. The problem
arises with external loads, such as older 240VAC amplifiers which assign
blowers, low voltage power supplies, and filament transformers to one
120VAC leg of the 240VAC circuit. Our point of disagreement is whether
we should connect the "third wire" in the amplifier power cord to N (my
opinion) or to G (K9YC's opinion). If everything is working properly,
the amplifier will work either way, and many users might not notice the
difference. That said, there are significant safety issues in favor of
connecting the amp to N and not to G. Here's my reasoning:
Consider first a simple 120VAC circuit that consists typically of a Live
120VAC wire (black insulated) a Neutral (white insulated) wire, and a
third bare wire (Ground). The L black wire and the N white wire are
identical, except for their color. There are two kinds of loads that use
120V circuits. One is for heavy loads and modern electronics (TVs, PCs,
printers), which use a three wire power cord. The ground wire connects
to the frame of the power tool or the chassis of the computer and in
normal situations never carries current. The entire load is between L
and N, and never L and G. Small appliances (table lamps, soldering guns)
have two-wire power cords, and always also connect between L and N. If
the 120VAC circuit has a ground fault interrupter (GFI), it looks for
imbalance between L and N, and if it finds any imbalance, typically a
few mA, the GFI assumes there is leakage current to ground and trips the
circuit.
Now consider a typical 240VAC circuit. In the breaker box, there will be
an L1 conductor (black), an identical L2 conductor (typically red), an
identical insulated (white) wire, and a fourth bare ground wire that is
often a lighter gauge. The desired load for this circuit is to use L1,
L2, and N to carry all the current, with no current flowing through G.
(G should still be hooked up, to a enclosure typically, but doesn't
carry current.) Note that if a pure 240V load is connected to the
circuit, all the current will be carried by L1 and L2 (which are out of
phase with each other). Because L1 and L2 are balanced (equal and
opposite), there is no current through N nor through G. In our
hypothetical amplifier, however, which has 120V components (e.g.,
filament xfmrs, blowers) and 240 V components (plate xfmrs), there can
be significant imbalance. In that case, the neutral wire carries the
imbalance current, but no current ever flows through the ground wire.
The problem is that older appliances and amplifiers have only three
wires in their power cords, so do we connect them to L1, L2, N or to L1,
L2 G? It is /always/ better to use L1, L2, N, because we know then that
any imbalance current will go back to the breaker box through
appropriately sized insulated wire. If we connect the amplifier to L1,
L2, G, we can't know how the current returns. Some of it may return
through the bare ground wire, but some of it may wind its way back to
the breaker box via coax shields or other metal frames and
enclosures./One should never rely on the vagaries of unplanned, random
connections to complete a circuit./ I learned that lesson the hard way
in high school when I unscrewed a PL259 from a homebrew amplifier and
suddenly discovered I was helping complete the current return path.
Note that if your house wiring uses a green insulated conductor for
ground, instead of a bare wire, then then you're probably on safe ground
(pun intended) using it instead of a white Neutral wire. The reason, of
course, is that the green and white wires are basically in parallel, all
the way from the electrical outlet to the breaker box, so it doesn't
make any difference which one you connect to. Even with this scenario,
however, you're relying on happenstance to complete the ground return.
That problem does not appear with modern amps, which keep N and G
separate at all times. In my own station, I connect the power cords from
my older KWS1, 30S1, Viking 500, etc., to the Neutral outlet pin and
always make sure I have a separate station ground connected to the
gear's metal enclosures.The external ground wire keeps things safe!
73,
Jim W8ZR
On 5/4/2021 2:44 PM, Paul Christensen wrote:
That's certainly true for the 87A being discussed here and most modern amps.
However, some older amps like my Alpha 70 and 77 series have the blower
connected between L1 and the chassis where in those designs the chassis
functions as both neutral and ground because of the blower wiring error.
The fix is actually very easy when 4-wire service is available. Not so easy
when it's not. Manufacturers did things like that back then. Although not
strictly a NEC violation, that practice violates many local codes and
product wiring standards since the wiring fault is inside the product and
not the premise.
In the Alpha 70/77, the blower is in series with the HV primary by design
which causes the blower to increase its speed when the amp is under full
load. If a 120VAC blower must be used and a neutral isn't available,
probably a better approach (like the Alpha 86) is to insert the blower
between L1 and a 240VAC primary transformer CT if it has one.
Paul, W9AC
-----Original Message-----
From: Amps <amps-bounces@contesting.com> On Behalf Of Jim Brown
Sent: Tuesday, May 4, 2021 4:08 PM
To: amps@contesting.com
Subject: Re: [Amps] Alpha 87A Power Supply Question
On 5/4/2021 7:24 AM, MU 4CX250B wrote:
I believe the appropriate hookup depends on the circuitry. As already
noted, the guiding principle is that return current should always be
through neutral, and never through ground.
NO, NO, NO!
As K4FMX has correctly told you, the amp needs ONLY Line, Line, and Ground.
It does NOT need Neutral, and you should make no connection to it. It is a
HUGE violation of Code to connect Neutral to Ground anywhere except the
Panel where power enters your home.
73, Jim K9YC
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