atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services Security, Alarms and Paging 70 volt questions

ok guys...is a 25 or 70v system AC or DC voltage? I always see it written as 70.7v RMS but I have never seen if it is AC or DC...or am I just confused?
-Jeff

Jeff I'm not sure which systems you're referring to, but RMS stands for Root Mean Square and is the formula used to determine the effective value of an AC sine wave. Just as an FYI that's why you want to good RMS meter like a Fluke as opposed to a cheap multimeter that just reads voltage by taking an average.

Jeff, it's audio so it's always AC and you want to measure it with an AC range on your meter.

As Trace says, RMS or root mean squared is the actual useful or heating value of an AC peak-to-peak voltage and is calculated by multiplying the peak voltage by 0.707. Unless peak or peak-to-peak is specified, AC voltages are assumed to be RMS.

Most good VOMs will measure the true RMS value, cheap ones will fudge it and not give accurate values if the waveform is distorted and deviates from a clean sine wave.

-Hal

I was always taught that RMS was to convert to the equivlent DC voltage. which is not really needed any more because there is not really any DC being distributed any more.

Jeff -

25 & 70 Volt Audio Systems are "constant voltage" systems that feed power to a Transformer (usually mounted on the speaker).

The transformer is the key to the answer. You can't transform DC.

Constant voltage systems are AC.

Sam

So am I correct in remembering that 70.7 is the peak measurement of 100 volts?

I was always taught that RMS was to convert to the equivalent DC voltage.

Yes, the RMS or "heating value" of the AC voltage would be equivalent to that DC voltage across the same resistive load which would make it give off the same amount of heat (power or useful energy).

Not sure how RMS came about but I suspect it was early on by the electrical industry as a means to compare AC to DC power as you suggest.

I should point out that multipying the peak by 0.707 to get the RMS value is only good for a sine wave, such that you can reasonably expect from a power generator. So this method adds credence to the roots of RMS measurements in the electrical industry.

For anything other than a sine wave there is a complex formula that looks at various points of the waveform vs time to arrive at an RMS value. Not something you would want to do, that's why they make true RMS meters.

-Hal

So am I correct in remembering that 70.7 is the peak measurement of 100 volts?

How about "am I correct in remembering that 70.7 RMS is 100 volts peak?"

What's 0.707 times 100?

Incidently, the 70.7 constant voltage system came about to get around building and electrical codes that put restrictions on voltages higher than 100 volts peak.

-Hal

Very interesting. I was not aware of that part. You learn something new around this place every single day.

"Incidently, the 70.7 constant voltage system came about to get around building and electrical codes that put restrictions on voltages higher than 100 volts peak."

Hal is again on the money. I started in this industry doing PA and that was what I was told. Here in NYC (and maybe elsewhere) they later changed the ruling that Class 1 wiring (circuits that require conduit or flex) included all circuits above 50 Volts. That was when 25 Volt PA circuits came to be.

In Europe were 240 is the normal current, PA circuits are usually 200 Volts.

BTW - When they changed the low end of the Class 1 wiring, why did they pick 50 Volts?

Because Telephone Circuits are nominally 48 Volts and they didn't want to open up that can of worms.

Sam