VT (PT) Output Voltage - why not Extra Low Voltage (ELV)

[DiscoP]

All our protection and metering equipment uses 63.5/110 Vac secondary (or 120 V in some parts)

In my part of the world, below 50Vac is considered ELV, and 110 V needs to be treated the same as 999 volts.

When I first started out 110 V was considered safe, but times have changed

There doesn't seem to be a lot of equipment you can buy with an ELV nominal voltage.

Are there any technical reasons for this, or is it just because it has always been done this way.

 

[waross]

Before digital low burden relays were developed, the electro-mechanical protection relays had a much higher burden.
The metering standards have been for decades, even generations, 5 Amp secondaries for CTs and 120 Volt secondaries for PTs.
63 volts may be a 120 volt output with the primary in a wye connection where a fault of some kind may cause the secondary voltage to rise to 120 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Of course there is a historical evolution component to this (eg. early filament light bulbs happened to work best around 110V), but fundamentally high voltages have great benefits for distribution. It becomes a tradeoff between safety and cost. Trying to distribute 50Vac for any appreciable number of loads becomes very wasteful (costly) due to I2R losses.

Each country has picked a voltage for distribution (415V/240V in Australia, 208V/120V in USA) based on this trade-off. Interestingly, both these examples are in the process of being lowered (to 230V and then to 220V in Australia, to 110V in USA). Because of these choices, manufacturers design their equipment to suit these voltages so they'll be compatible.

There are some examples of attempts to change this. Sometimes 110V is distributed as a split phase 55-0-55 system. 55V is so much closer to ELV than HV that the UK has implemented a "Reduced Voltage" standard to accommodate 55Vac.

On the other hand, many devices transform the supply voltage to a lower voltage internally anyway. Each individual device may happily run from a lower voltage, but as soon as you distribute them, or put several of them on the same supply, the lower voltage becomes more of a problem. So in summary - LV better for distribution -> power points are LV -> equipment designed for LV. When you can control the environment (eg. medical operating theatre, hazardous zone in underground mine) then you can control the distribution voltage, and you'll find plenty of equipment that runs off ELV.

 

[davidbeach]

LiteYear, I think you missed the point. The question is about instrument transformer secondary voltages.

 

[LiteYear]

I think you might be right davidbeach. I took the statement "There doesn't seem to be a lot of equipment you can buy with an ELV nominal voltage." very generally. Restricting the question to instrument transformers is more interesting. I await some insight!

 

[DiscoP]

Thanks for the comments so far. A good point about burden, which would have been an issue in the past.

Just for clarification - I am only asking about instrument loads, and 63.5 is the Ph-N voltage of a three phase 110 V rating.

Sorry for any confusion.

50 Vac / 120 Vdc is the ELV to LV threshold in our standards.

The substation battery is ELV, therefore most test terminals on our panels are ELV - except the VT ones.

 

[MacGyverS2000]

Interestingly, both these examples are in the process of being lowered (to 230V and then to 220V in Australia, to 110V in USA).

What do you base this statement upon? The only time I ever see 110V is when the socket is far away from the local PoCo... (fairly stable) 120V +/- seems to be the norm everywhere I've seen.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[waross]

(to 230V and then to 220V in Australia, to 110V in USA)

Interesting.
When I was very young, I experienced the last years of a 20 year project in North America. For about 5 years we saw appliances rated at 112 Volts instead of the then common 110 Volts.
Then 5 years at 115 Volts.
5 years at 117 Volts.
Then about 40 or 50 years at 120 Volts.
I will be watching for 117 Volt rated equipment as the first step in this reversal of standard voltages. (Or may be 118 Volts going the other way.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LiteYear]

Off-topic warning...

What do you base this statement upon?

Ha! From a misreading of

http://en.wikipedia.org/wiki/Mains_electricity_by_country

I thought that list said 110V, and put it together with similar (though not exact) moves in the UK and Australia. It seems there's no such trend in the States. Mea culpa.

Incidentally, while the trend in the UK is more to do with harmonisation with Europe, the trend in Australia is at least in part due to (somewhat questionable) attempts to lower energy use. There is currently a surge of "Voltage Optimisation" (VO) products that purport to save energy by lowering the mains at a site to 220V. Some customers (typically those with lots of 220V rated air-conditioning motors) have indeed experienced savings. These efforts have coincided with a huge national interest in a "Smart Grid", which encompasses just about any modernisation technique you could think of if you were to overhaul the electricity grid. One technique being discussed is to lower the supply voltage and take advantage of better regulation out to the customer. So instead of having to start at 250V so you can manage 230V at the further customer, you start at 230V and only drop to 220V at the further point.

 

[dpc]

The ANSI standards that govern standard voltages in the US have not changed for a long time, IIRC.

The OP question about lower PT standard voltages is interesting. With modern digital relays and meters, it would seem possible, but it would be another 100 years before existing equipment got changed out. High voltage PTs are expensive.

I remember seeing an ABB presentation about 15 years ago promoting their optical current sensing technology - the Soupy Sales guy stated that conventional CTs were obsolete and that they would soon be out of production. Still waiting on that...

I recall a push in the auto industry in the US to convert their motor control center control power from 120 V to 24 V dc to reduce shock hazards. That seems to have not come to pass - or has it? Of course the auto industry was also talking above a change in auto batteries from 12 V to 48 V - not sure what is happening with that.

AFAIK, the NEMA standard for MCCs still allows for 480 V pilot devices (pushbuttons, selector switches, indicating lights) on the front of the MCC bucket door. Control power transformers are an extra cost option.

 

[waross]

A rule of thumb developed years ago when solid state logic systems were introduced was that it took about 100 Volts to reliably puncture contact films. There have been a lot of isolated OEM systems using lower control voltages but the main stream of industrial installations seems to be 120 Volts control.

Bill
--------------------
"Why not the best?"
Jimmy Carter