[RFI] Daisy-chain grounding [was DigiKeyer II]
EDWARDS, EDDIE J
eedwards at oppd.com
Fri Dec 20 13:06:27 EST 2013
Actually, you are correct in the sense that the R-56 grounding guidelines are not based on circuit board designs, but they are based on real world problems and experience on lightning protection at commercial transmitter comm sites so a different set of "circuit fundamentals" are applied for a different signal source. As they say, one size doesn't always fit all for differing electrical circuits. AC power circuits are based on different fundamentals from RF circuits or from audio circuits depending on the signal used. And RF ground is different from AC safety ground, etc...
Motorola expanded their original R56 guidelines from around 50 pages to 518 pages in 2000 in response to repeated equipment failures at radio sites related to poor installation practices when following their old R56 booklet. Lightning is not equivalent to a DC circuit or even a more complex signal like an audio or data circuit. Lightning protection, as I understand it, is based on creating a single current path with the lowest possible "impedance" for a lightning strike covering from DC to RF energy.
The theory is "daisy-chaining" or series connections that create multiple connections on each device can create impedance "bumps" increasing overall impedance all along the chain. Even if only two wires are connected to the exact same point on a device, it is considered a "multiple-connection", i.e. more than one conductor/wire making contact. Lightning will find the path of least impedance (not least resistance) so these little impedance bumps might cause the lightning path to go somewhere unexpected like through some portion of the devices' circuit boards instead of the desired path. This is especially true if the multiple ground wires are incorrectly connected to multiple locations on the same device leaving a new possible path through the device's circuits instead of around it.
Another problem with daisy-chaining in commercial set-ups is what happens when a technician temporarily removes one of the devices in the middle of the daisy-chain for repairs. The result is most of the other devices are left unprotected until the missing device is returned to service. You just cannot predict what service techs will do on a given day, but especially on a Friday! ;-)
When Single-Point grounding is done correctly, it will greatly reduce (but not eliminate) the chance of lightning damage to equipment that operates 24-7. Motorola expanded use of single point grounding requirements with Motorola's R-56 in 2000 (updated in 2005) which can be found on-line at this link: (see Chapter 5 section 5.4.1 page 5-33,34 for specific daisy-chain info): http://www.radioandtrunking.com/downloads/motorola/R56_2005_manual.pdf
M/A Com (back in 2001) also had or has a shorter 56 page version (ironically numbered), but also requires single-point grounding that meets the National Electric Code (NEC). (See http://www.repeater-builder.com/ge/lbi-library/t4618r3a.pdf see section 1.5.1) (Note: The M/A Com guide still refers to use of an internal comm room "Halo" which has been abandoned by most vendors today.) I believe NEC also forbids multiple wire connections (similar to daisy-chaining) for safety reasons in electrical circuits.
All that being said, I'll repeat what I said originally: The typical or simple ham radio station can probably get away with some very short-runs of daisy-chaining (especially on computers) since most hams have human-operated layers of lightning protection such as disconnecting all antennas and grounding all the cables at the entry port during possible thunderstorm periods. If a ham does all that, he reduces his risk of the daisy-chaining having any impact.
On the other hand, a multi-multi contest station should probably employ the same single-point grounding standards that the commercial guys do whenever possible. Also, I would not daisy-chain computers and RF devices together, but keep them separate if at all possible. Physical equipment layout to separate RF devices from computers would make this possible. I keep my computers on station left and RF on station right or middle with cable entry from the right.
QST has had some good primer articles on lightning protection and grounding for ham shacks: a search for lightning in ARRL's Periodicals Archive found an article in 2002, June & July. I know there are others more recent.
73, de ed -K0iL
From: RFI [mailto:rfi-bounces at contesting.com] On Behalf Of Jim Brown
Sent: Thursday, December 19, 2013 4:20 PM
To: rfi at contesting.com
Subject: Re: [RFI] DigiKeyer II
On 12/19/2013 1:39 PM, EDWARDS, EDDIE J wrote:
> But for anything connected to RF equipment, daisy chaining violates the "guidelines" and specs for single-point grounding systems. This may be acceptable as long as the overall shack has proper lightning protection in all other cases.
That is NOT correct. These so-called guidelines are based on a
misunderstanding of circuit fundamentals.
73, Jim K9YC
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