[clug] Telephone line voltage

steve jenkin sjenkin at canb.auug.org.au
Sun Sep 5 18:45:25 MDT 2010

Brian wrote on 6/09/10 4:14 AM:
> 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

A Telstra tech tested this line with their box-of-magic, and it doesn't
flag any AC leakage. He was at a loss to explain or interpret the
multimeter reading. [Beyond his training, I think.]

Two different electronic multimeters have been used on the line.
Both detect ~48V DC, both detect ~100V AC, mostly but not always.

I've used my multimeter on my home line - same deal.

Could ADSL RF frequencies (or other signals) be inducing apparent
high-volt AC in the multimeter??

I can't explain:
 - not always there (but mostly)
 - Telstra can't explain and isn't concerned. [Would you expect less]

I'm inclined to think it's an artefact of the way modern electronic
multimeters work... But that's not an explanation.

I believe Keith chased Telstra further, to no avail.

Steve Jenkin, Info Tech, Systems and Design Specialist.
0412 786 915 (+61 412 786 915)
PO Box 48, Kippax ACT 2615, AUSTRALIA

sjenkin at canb.auug.org.au http://members.tip.net.au/~sjenkin

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