Electric Zap in Ear from 7940

Unanswered Question
May 10th, 2007

3 times over the past 4 years, we've had staff be "zapped" in the ear by their 7940 phone while using the handset. In one case, the employee went immediately to the hospital, in all 3 cases the employees experienced pain for several hours. The first time this happened, TAC said our grounding at the data-rack must have been bad. We've had electricians triple-check the grounding and all is code. Since this happens so rarely and can't be deliberately reproduced, it's obviously hard to troubleshoot. The only things I can think of to do is to give the latest victim a new switchport on the switch and a new telephone set. The switch model we are using is a powered 3550. Has anyone ever heard of this before? If so, any ideas?

I have this problem too.
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rob.huffman Thu, 05/10/2007 - 09:14

Hi Keli,

This is a fairly common problem, especially in very dry climates. The problem you are describing is "Static Discharge" and we have had it happen to a few users as well. Usually it Zaps the Power Module (PoE) in the associated switch as well as the persons ear :( This has happened on every Phone system I have ever worked on not just the Cisco IP Phones. The user is almost always someone working in a carpeted area with Leather soled shoes or a carpeted area and rolling around in their desk chair. As they walk/roll on the carpet Static Electricity builds up (to feel a static discharge/shock it is up to 10,000 Volts) and this static is discharged when the Handset touches the ear. If this person had touched something other than the phone first this discharge of static would not be felt/sensed.

Usually some type of "User Awareness" information will alleviate the problem, but I have had users over the years where we had to install Anti-Static Floor Mats and Desk Mats to fully eliminate the problem.

Here is an example of these mats;


Hope this non-scientific explanation helps!


kelimeyers Thu, 05/10/2007 - 09:33


thanks so much for the quick reply! In re-reading what I wrote, I realized that I forgot to mention a very important fact: the phone reboots after the "zap". This is consistent with your explanation since it zaps the poe module, right?

Thanks again!

kelimeyers Thu, 05/10/2007 - 10:19

I've spoken to the user again who says she has an anti-static floor mat and that she was zapped in the middle of her conversation; do you think a static discharge is still the answer?

rob.huffman Thu, 05/10/2007 - 13:39

Hi Keli,

I am still 99% sure that this is from Static Discharge. You would have to check that this users Anti-Static Floor Mat is properly grounded and probably you would be well served to provide an Anti-Static Desk Mat as well. The chances of this shock having come from the phone system itself are miniscule. The fact that this person already has an Anti-Static Floor Mat would lead me to beleive that she has had this type of problem before. Some people are just more "Staticky" (is that a real word??) than others. Teach people to take a moment to discharge themselves by touching the mat before touching the phone. For those on the phone already touch the mat if they have been rolling around (which I would bet this user was).

Here are a couple of interesting articles to have a look at:



Hope this helps! Just do the best to provide a good environment :)



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