Unusual 3 phase 240V supply encounter - Page 1
Forum section: electronics / projects

author: Pudd74
author_rank: Newbie
timestamp: January 21, 2020, 12:03:09 pm
content: G'Day all,First time here, thanks for having me. Sorry, for the long post first up. Hoping someone else might find this interesting, or just a lesson on traps for young players.I went out to a service call today, to take a look at why a variable speed (BLDC) pool pump wasn't working after the huge hail & thunderstorm that we had yesterday afternoon. So, the first thing I do is look at the LCD display on the saltwater chlorinator (which the pump plugs into, and is switched from), it was lit and showing "Pump Running". The LCD display on the pump was not lit, no sounds, no smells, no working. Unplugged the pump from the chlorinator and tried it directly into the power point, still dead. So, from "experience", I know that if there's power at the power point lighting up the chlorinator, then no reason why the pump shouldn't work.Was just about to start pulling the pump out for bench testing, when old mate from across the road comes over to borrow a chainsaw off the neighbour, and we get talking. He tells me that since the storm only some of the things are working in his house, some lights are dim, some don't work at all, some tv's working etc. He tested his voltage (clever guy) and was getting between 60 and 80V supply. So I pulled out my meter and was getting the same fluctuating 70'ish volts on the pool pump/garage circuit. Tested a point inside the house (which is on a different phase) and got 245V on the meter. On the garage and pool circuit/phase that I tested at 70V, LED downlights lit, the spot light was a bit dim, and the fluoro tubes didn't come on at all.So as a pool service guy (with more than average experience working with electricity, electronics, Arduino - an enthusisast working without a parachute) I have a few questions that I know a lot of you wise folk may be able to speculate on.I am going to assume that the 70V that I tested on that one phase was a true reading. Because that would explain why devices using switch-mode power supplies rated 110 -240V are still able to just operate, and why LED lights are just working. I did not think to check the frequency at the time, now I wish I did.So, what would cause this 60-80V to appear on this 240V phase? Lightning strike on the transformer takes out one phase partially? Ghost voltage, with enough oomph to power a few LED's, from some induction in blown transformer?Would leaving "Things" turned on in this low voltage state stress them (or components within) - thinking computers, phone chargers, induction motors, etc.It seems to me "weird" and wrong that Ausgrid would leave this phase connected if they knew it was only putting out 70V.It was interesting to see how different "devices" dealt with the low voltage supply. A lesson in not making assumptions, taking the time to listen to old mate next door, before taking the pump home and making an arse of myself.Thanks in advance to brains trust.Mark

author: BradC
author_rank: Super Contributor
timestamp: January 21, 2020, 12:11:24 pm
content: Perfectly common fault condition. Might be isolated to the line segment or more widespread. A common reason for toasted induction motors and a good reason most industrial stuff has phase monitoring to disconnect the load if a dropped phase is detected.We had one through our place a couple of years ago. Took out the fan motor in the neighbours A/C. Ours was on one of the other phases and just kept going.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 21, 2020, 12:14:41 pm
content: Very typical of a weak or broken neutral line at the local transformer. (Assuming that 245V is a rather high value.)Running low could stress badly designed SMPS pushing them to high duty-cycles.I'd be more concerned by the voltage being too high. If it is?

author: richard.cs
author_rank: Super Contributor
timestamp: January 21, 2020, 12:25:16 pm
content: A broken neutral could do that, with the consequence that at least one other phase has an unusually high line-neutral voltage. Measuring the voltage between neutral and true earth (e.g. a stake in the ground outside) should reveal this. If you suspect a broken neutral you should disconnect everything immediately as the load balance may change at any time and result in destructive line-neutral voltages.Another possible fault would be an open circuit HV phase prior to the transformer. The delta primary "flattens out" and on the low voltage side you get one phase working as normal and two at half voltage 180 degrees apart (because those two primary windings end up in series). 80 V is a bit low for that though.

author: tautech
author_rank: Super Contributor
timestamp: January 21, 2020, 12:36:19 pm
content: Typically it's a dropped HV phase at the powerco's transformer. 2 HV phases still working supply others on the same transformer just fine and only those on the same LV phase will be affected. Seen this here a few times over the years after storms and a mad rush around the house ensues to turn OFF voltage sensitive devices like pumps. fridges and freezers.

author: station240
author_rank: Unknown
timestamp: January 22, 2020, 07:23:55 am
content: Yup it's a brown-out, caused by a fault somewhere on the HV side of the street transformers.Bad cable, faulty insulator or transformer.Sometimes the power company also have to inject a lower voltage into the system to find the fault.HV thumper in use