At BBC training college, we were told the story of the Ulster power grid using a TV looking at a mainland transmitter to phase things up before linking into the UK grid. When the hum bar became stationary, hit the button. ;-)
Of course like all such things, likely a myth. ;-)
I believe there is something similar for determining when a portable generator is in phase with, as well as just running at the same speed as, the mains, and therefore it's safe to switch over. It's something to do with connecting a neon from mains live to genny live (assuming neutrals are permanently connected) and waiting until the neon goes out so the two lives are in phase and so there's no voltage between them.
The hum bars drifted up and down depending on the national grid supply frequency. You could also observe this effect on 625 line TV sets back in the 80s prior to the advent of 24 hour TV when it was possible to observe them against a static grey background after the last scheduled TV programme of the day, before the transmitters were shut down, if they ever were actually shut down overnight by then.
And we're never going to let him forget that fact, are we lads! :-)
Although I've observed the clipped sine wave effect on the mains supply using an ancient valved CRO "Boat Anchor" some 15 to 20 years ago and have subsequently seen the same effect more recently using CoolEdit Pro to record the mains waveform via a step down transformer about 5 or 6 years back, I've only ever observed it as a symmetrical effect.
Incidentally, my initial description of the flat tops having a negative slope towards the zero crossing baseline turns out to be the inescapable consequence of using AC coupling.
When, about a month back, I finally got hold of a modern DSO that gave me, for the very first time, the option of DC coupling, I could see the sloped clipping effect had been an artefact of the HPF effect of the AC coupled signal path I'd had to rely upon to see the waveforms.
Observing with the new DSO using the AC coupling option showed the same artefact of a down sloped flat topping appearance until I selected the DC coupling option whereupon the waveform magically morphed into a clipped sine wave with perfectly level tops.
Considering just how low the HPF turnover frequencies are for the DSO and sound card (2 and 8 Hertz respectively - Ghod knows what it is for the ancient Boat Anchor CRO!), it's surprising just how exactly the same this down sloping effect appears.
Quite frankly, with an HPF turnover frequency as low as 2Hz on the DSO's AC coupled signal path, I'm surprised the effect was still so observable. Who would have thought such a thing was even possible with such a low cut off frequency as this when observing a 50Hz wave form?
Off air cues for live TV these days are not really practical. The round trip time from a live OB back to the OB off air is around 8 seconds...
In article snipped-for-privacy@brightview.co.uk>, NY snipped-for-privacy@privacy.net scribeth thus
Yes thats how its done usually with a 240 odd volt lamp..
====snip====
If you're referring to the case of syncing up two 240 volt generators (national grid versus an on site generator or two on site gensets), you're likely to blow a 240v filament lamp when the two sources are completely out of phase (480v difference).
I think the practice of using filament lamps involves the use of a pair in series to get around that problem. Also, since the lamps will become too dim over a wide band of phase discrepancy, it relies upon the operator's judgement as to when to operate the circuit breaker to combine them together.
It's not quite so critical if it's just to cut back to the mains supply after an episode of emergency generator power in order to minimise the switching transient effect of a rapid changeover back to the mains supply.
UPSes delay the cut back to mains power at the end of an outage, not only to establish that the supply has been restored to normal but also to allow it to shift its inverter frequency towards that of the incoming supply so it can effect the changeover in synchronisation with the mains waveform at the zero current crossing point[1] to likewise minimise the switching transient to that of the inescapable 4 or 5 millisecond's interruption during the changeover switching period.
[1] This won't necessarily be at the zero voltage crossing point if there is a significant reactive component in the protected load.HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.