Hi all the term medium voltage has as far as I know been out of favour for a good few years i think I'm right that any thing up to 999 volts is LV and anything above that is HV is this still the situation? I ask because I've seen a few videos and Web pages recently describing medium voltage switch gear and equipment these are mostly pages that keep coming up now following my searches on power factor correction. Which in turn were a result of my post on large cotton reels in substation you now how it is a few mouse clicks and your into something else
Here in BSI and the Euronorm ISO land, you are quite correct the phrase 'medium voltage' has no definition, we go from LV below 1000V RMS or 1500VDC to HV above that. (it is the higher of the phase to phase and phase to neutral voltage, so 690/1k2 3 phase is also considered HV but a single phase of 690 to earth would not be - not that I've ever seen that used in the UK, only in Germany. Here we seem to have LV as 230/400 3phase and occasional flights of LV at 400/690 or jump to full blown HV and run at 3k3, 6k6 or 11k or something.)
However in the US and places that adopt their standards, you will find MV referring to the layer between about 1kV and 30-50kV.
So our 11kV and 33kV lines, that are HV in the UK would be called MV in other places. As the makers of insulators and switches for these voltages sell similar parts to both regions, you will find literature that uses 'the wrong words' is also in circulation
Absolutely agree with Mike, but should add that if you come across the term Medium Voltage the upper limit between MV and HV varies depending on where you are in the world so you need to make sure you know what range is being discussed.
Thanks both I think the website i saw was from the US so that makes sense. Also there's the thing that in the electronics world the terms HT and LT are used I've seen sometimes that even 12 volts is referred to as HT so it can get a bit confusing thank goodness the electrical world has a sensible demarcation between levels
Also there's the thing that in the electronics world the terms HT and LT are used I've seen sometimes that even 12 volts is referred to as HT
Tension or Pressure used to be considered alternative descriptions to voltage - LT/HT is still common in the automotive industry for example. I'd be surprised at 12V being referred to as HT though - you sometimes come across it in cars but it'll usually be referring to the ignition system - traditionally supplied by a transformer (spark coil) and so would itself be at several thousand volts even though it's supplied by 12V.
Right at the end of the valve era, in a desperate bid to keep pace with semiconductors and the miniaturisation they offered, very small high gain frame grid valves ('large' frame grid valves ) were made, exploiting all the photo-resist and etch techniques of the PCB lab to get really small clearances, so the anode voltage (HT) could be made low enough to compete with transistors of the day.
Even before that the 1957 issue of the Mullard Technical Communications magazine shows a reference design for a car radio using the specially made low voltage heptodes ECH83 for the RF front end and an OC16 transistor for the audio output. (link - here the HT is 12-15V depending on the car dynamo.)
Actually if you do not mind micro ampre anode currents and high impedances then even the traditional large valves can be made to work, albeit with really low gain, at much reduced HT - it is also quite possible to demonstrate thermionic diode operation at 12V with a headlamp bulb with one blown filament, or the dual brake stop lights. (use the connection to the blown part as the anode, and the bit that works as a cathode. Do not expect more than a few tens of microamps though.)
