Often in practice we have found in our lab facility, multiple loops caused
by bonding green safety ground to neutral in multiple boxes that may exist
between the service entry and the ultimate site of the powered instrument.
Failure to remove the neutral - safety ground bond in electrical sub-panels
is a violation of the National Electrical Code requirement for a neutral -
safety ground bond only at the service entrance panel. Failure to remove the
neutral - safety ground bond in electrical sub-panels may not only cause
electrical noise, it can result in sub-panel circuit breakers failing to
operate (I've seen #12 hot wires burn in two while performing electrical
maintenance) and the main breaker tripping out at the service entrance
panel.
On Sep 2, 2014, at 12:31 PM, "Bob McGraw - K4TAX" <RMcGraw@Blomand.net>
wrote:
You raise a point regarding "industrial grade sine wave ups". I have one
of those and in my chasing a noise issue, I opened the case to find the
output receptacles each have the neutral terminal {white wire} and the 3rd
pin ground terminal {green wire}connected by a solid strap. This then
says the neutral is connected to 3rd pin ground at several points, both on
the input and output. I seem to recall this is not correct according to
NEC.
The above situation (the output receptacles each have the neutral terminal
{white wire} and the 3rd pin ground terminal {green wire}connected by a
solid strap) is NOT in violation of the NEC as long as the input neutral
(white) is not grounded to the input ground (green / bare). The
manufacturer of the UPS may have considered the neutral - safety ground bond
to be necessary for safe operation of standard three wire plug supplied AC
devices, not considering the almost standard chassis supplemental grounds
of lab and communications devices. When you removed the strap you most
likely removed the ground loop from the isolation transformer.
(The things I remember from my time doing electrical maintenance at State
Farm Insurance Corporate Headquarters and radio and telecommunications
applications development for State Farm Mutual)
73 & DX,
Gary - AB9M
-----Original Message-----
From: Stuart Rohre
Sent: Wednesday, September 03, 2014 2:52 PM
To: tentec@contesting.com
Subject: Re: [TenTec] TT 940 Power Supply
As a follow up to Bob's comment about checking for continuity between
safety ground terminal on an outlet and neutral; it is my understanding
from current codes, written up in electrical trade magazines
discussions; that the neutral is bonded at one place to the safety
(green) ground wire, and that is at the breaker box, presumably at the
entry to the facility.
Bonding in the UPS box seems counter to this practice, and certainly
creates a loop between neutral and safety ground.
Often in practice we have found in our lab facility, multiple loops
caused by bonding green safety ground to neutral in multiple boxes that
may exist between the service entry and the ultimate site of the powered
instrument. It caused bad noise problems such that I finally had a
special feed conduit installed that was bonded at its source of power
but was insulated on brackets until it reached the lab needing low noise
A/D wiring. That finally met our noise spec. There was still
continuity from neutral to safety ground, but we controlled where that
singular bond was made.
-Stuart
K5KVH
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