Question for Smurf when the system on the East Coast gets overwhel who gets cut off 1st industry or domestic, is there a choice?
For an unplanned incident, the kind of thing that would occur if a major generation failure unexpectedly occurs, UFLS (Under Frequency Load Shedding) will disconnect load in blocks once frequency thresholds are breached.
In theory that should work in a pre-determined order, but you won't find anyone willing to state categorically that it'll always go to plan. Depending on the magnitude of supply loss, in practice load disconnection can end up somewhat of a mess.
For a planned load shedding, or for restoration of supply after an unplanned trip in a situation where it's not actually possible to put everything back on (due to an overall lack of available supply) there's really two stages:
First is RERT (Reliability and Emergency Reserve Trader) which in simple terms are industrial loads and private generation (eg backup generators that aren't actual power stations and which aren't normally operated - eg backup diesels at TV stations for example). This involves payment to disconnect load or to generate.
Beyond that it's involuntary load shedding as such or in industry terms an LOR3. In layman's terms that's blackouts.
What load to shed, and in what order, is based on a pre-determined list set by government. Whilst the requirement is a technical one as such, the priority order isn't important in a technical sense and is thus a political decision.
SA's load shedding list is public and is here:
https://www.energymining.sa.gov.au/__data/assets/pdf_file/0005/852827/Manual-load-shedding-list.pdf
A key point about the SA list being it doesn't reset to No.1 on each occasion but rather, continues from the most recent incident. So if load shedding were to become routine, everyone on the list would be affected equally over time - the only ones who wouldn't are those not on the list at all.
Other states it's strictly confidential but broadly speaking, the lowest priority for load shedding (highest priority to maintain supply) will be hospitals and next comes the CBD, important infrastructure such as water treatment plants and so on. Any industrial facility that'll suffer equipment damage or cause a pollution incident will also be a priority to maintain supply to.
Note regarding industry - there are examples of facilities which are effectively split into two loads, one that's much higher priority than the other, at the same site. The high priority being the one that, if interrupted, will bring about serious consequences and the low priority one being the rest of the plant. That discrimination being done on site with the UFLS settings and, in the case of manual load shedding, operating procedures. Physically both groups would typically be supplied via the same physical transmission lines so it's about turning the low priority one off on site, not about it outright losing supply as such.
So broadly speaking the order is:
Voluntary industrial load and those with their own backup generators (RERT).
Non-harmful industrial load.
Mass market consumers - residential, general business etc.
Aiming to avoid critical infrastructure, industry where major impacts will result, plus the CBD.
And definitely aiming to not disconnect hospitals and anything else deemed particularly critical.
Note for reference that traffic signals are nothing special, they're simply fed from the nearest supply and will get the same level of reliability as the general area (so CBD higher than suburban). A minority do have backup, there's a battery backup system at the intersection, but for most if mains is lost then the lights simply go out. Rail signals have backup however. At least I'm not aware of any that don't.
Putting that into practical terms:
Minor shortfall = RERT can likely contain it.
Medium shortfall for an hour or two = industry will likely contain it if in Qld, NSW, Vic or Tas. In SA it would spread to residential and commercial.
Larger shortfall = it's going to hit residential and small business no avoiding it.
With all bets being off in the event of an actual system collapse. In that case even hospitals may well be load shed as there's simply no choice and no time to react.
A complication in all that being what happens with a serious shortfall over time:
Industry that can load shed once can't do it again until x hours (typically at least 24) have passed since the conclusion of the last load shedding, which precludes doing it at the same time each day.
Batteries are short term generation no more than 4 hours.
Diesel generation that can keep running only if adequate fuel is delivered since on-site storage isn't that much.
etc.