Actually, "sealing current" was not used for relay contacts.
Sealing current is still used today, in the time of digital telephone
switches (with no relays).
One evening over coffee at a technical standards meeting in Geneva around
1982, a Bell Labs engineer discussed this subject. As
best as I can recall:
What sealing current was designed to do is to ensure that connections made
in the outside wire pairs between the telephone
central office and the subscriber continued to function properly. A telephone
actual doesn't require ANY current to flow when the
phone is not in use. But telephone engineers learned early on that connections
tended to oxidize or otherwise form non-conducting
thin films when a wire pair was left idle (no current flow) for a long time.
Such films were often able to be punched through by an
incoming call, as a significantly higher pulsing (and alternating) voltage was
used to ring the phone. But if the subscriber picked
up the phone to originate a call, the line would appear to be dead.
As a result, the network was designed to 'leak' a little bit of current all
of the time at the distant termination of wire pairs.
This was the connection sealing current.
As phone networks became larger, this sealing current on thousands (or tens
of thousands) of wire pairs added up to a lot of
power and expense. Designers got clever and put a blocking diode in the leaky
part of the distant termination. The central office
equipment would apply voltage in the normal orientation to the wire pair ...
and no current would flow with an idle line ... but
occasionally (about once every day or two) the voltage feed was temporarily
reversed to cause sealing current to flow briefly. Some
field studies in the late 1960s/early 1970s had shown that sealing current
needs to be occur only once every few days to avoid the
thin film problem (to a high level of reliability).
-- Eric K3NA
-----Original Message-----
From: towertalk-admin@contesting.com
[mailto:towertalk-admin@contesting.com]On Behalf Of Jerry Kincade
Sent: 2002 March 28 Thu 16:12
To: dhearn@ix.netcom.com; Logan Dietz
Cc: towertalk@contesting.com
Subject: Re: [Towertalk] Relay Summary
Ma Bell used that principle with what was called "sealing current" to help
keep their gazillion relays working for many, many years. Now, of course,
it's all digital switches in the central office, but back then whole crews
of people were dedicated to relay maintenance, which was very labor
intensive and costly. The noise level from all those thousands of relays
stepping and clicking in a central office as calls were dialed and routed
through was amazing. Now it's just a low "hmmmmmmmm" from blower motors. :-)
The "wiping action contacts" idea certainly makes sense.
73, Jerry W5KP
----- Original Message -----
From: "dan hearn" <dhearn@ix.netcom.com>
> In fact there was no material that would guarantee total small signal
> reliability.They talked about bleeding a small current thru the contact
> via a large resistor or RFC to help punch thru the contaminent film.
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