[clug] Telephone line voltage

[Brian]

 From memory the telephone system is 48V DC with the center grounded ie 
it is +24VDC & -24VDC with respect to ground.

I once had a problem where the mains earth in a factory had a high 
resistance connection to earth instead of a very low resistance connection.

This fault condition made the factory earth float to half way between 
the active & neutral mains voltages because of leakage capacitace ie 240 
/ 2 VAC or about 120 V AC.

If you then connected to a good earth ie the telephone to the bad earth 
factory then it appears that one of them is 110V.

So the telephone earthing could be good but your house earth could be bad.

Brian

On 05/09/10 19:41, Keith wrote:
> On Thursday 02 September 2010 12:12:42 a startled Andrew Boyd wrote:
>
>    
>> Shocking!
>>      
> 	Indeed - at least it startled me.
>
>    
>> Puns aside, is this acceptable?
>>      
> 	Well I think not but Telstra seem to think it is.
>
> To which Sam Couter added :
>
>    
>> The short answer is yes, 110V is quite dangerous and can easily be lethal.
>>      
> 	To people or equipment?  All this started with trying to diagnose reason(s) for a failed
> motherboard and a failed answering machine.
>
> Jason perturbed further with :
>
>    
>> Get?even the lower bound?across your heart and?you have a heart attack.?AC is
>> particularly good for inducing fibrillation.
>>      
> 	And just on a year ago I had atrial fibrillation - but it was almost certainly due to post-
> operative stress.
>
> And a doubtful Mike Carden added :
>
>    
>> I'm a tad skeptical about 110VAC being present on a phone line for any
>> length of time.
>>      
> 	Well today (Sunday) I retested and with the black probe on the left, the red probe on the right
> and the test meter set to 200AC I got a reading of 107.4.  Switching the probes round the reading
> was nil so polarity may have something to do with it.  Then setting the meter to 200DC produced 49.0
> either way round.
>
> 	It still all bothers me, any suggestions would be welcome.  And anyone passing through
> Charnwood would be welcome to come and see for themselves.  Incidentally those reading were taken
> with all instruments disconnected - just a lead from the skirting board box up to my table where I
> had it taped down.
>
> -------------------------------------------------------------------------------------------
> Keith Sayers                                        keiths at apex.net.au
> 6 Clambe Place,
> CHARNWOOD, ACT 2615
> Australia                              http://www.apex.net.au/~keiths
> --------------------------------------------------------------------------------------------
>