Can you help me understand why a sump pump w/ shorted windings doesn't electrocu - Page 1
Forum section: electronics / beginners

author: TT_Vert
author_rank: Regular Contributor
timestamp: March 07, 2021, 04:32:21 pm
content: This pump is not on a GFI nor does code say you have to. My understanding is these pumps actually trip them. M I have an oil filled sump pump that I tore apart to do a post mortem on. I noticed some of the windings were burned and high resistance. To the tune of i think 200K ohms for one set of windings. It appears this 110Vac pump has line one one side of the windows and neutral on the other. This pump is one consecutive set of windings w/ just what appears to be a thermal cutoff inline half way between the (I think 6) sets of windings. So if the windings are compromised and exposed to the oil what prevents live shorting to ground/neutral and electrifying the water if the breaker does not trip? I noticed there was a bit (Maybe 1 teaspoon) of water mixed in with the oil when i investigated the internals of the case also. I know that would be conductive even if the oil isn't right?Sorry for the basic question, this just has me wondering why/how?Dave

author: SilverSolder
author_rank: Super Contributor
timestamp: March 07, 2021, 05:57:07 pm
content: One thought is that oil is an excellent insulator and it does not mix with water (it will at best make an emulsion, i.e. droplets of water suspended in the oil - think vinaigrette salad dressing). It isn't going to be a great conductor of electricity.Another thought is that a damaged winding could make direct contact with the metal case, but that ought to just blow the breaker - the pump is presumably grounded via a 3-prong plug?

author: TT_Vert
author_rank: Regular Contributor
timestamp: March 09, 2021, 03:15:19 pm
content: Yeah it was grounded and the breaker did not pop which confused me. So I'm guessing enough water hadn't intruded into the housing to short to ground and pop the breaker? How do they deal w/ this w/ water filled pumps give the conductivity of water?Dave

author: SilverSolder
author_rank: Super Contributor
timestamp: March 09, 2021, 03:53:03 pm
content: The motor could have failed in a way where the windings never contacted ground, so there would just have been a small leakage current that was too low to trip the breaker.GFCI relays are not a preferred option for sump pumps because you risk a small, insignificant electric leak shutting down the pump and destroying your basement... the preferred method to protect humans from sump pump electrical failures is to make sure the pump is grounded (and ideally, cabled in grounded metal conduits), so any leakage returns safely to ground without humans becoming a part of the circuit.

author: TT_Vert
author_rank: Regular Contributor
timestamp: March 10, 2021, 03:06:17 am
content: Thanks you. It's definitely grounded and run in metal conduit to the box. I was just curious. Also curious how this works w/a water filled pump. The water (Which definitely contacts the case housing which is submerged in the well has to contact AC right?Dave

author: SilverSolder
author_rank: Super Contributor
timestamp: March 10, 2021, 03:22:39 am
content: Yes, if the pump gets water filled, current can escape out of the windings, through the water, and into the metal case of the pump. From there, it heads straight into the ground wire, and makes its way back to the distribution panel via that wire. So no dangerous voltages are ever presented on the outside of the pump, even if it is filled with water.Countless lives have been saved by the invention of grounded appliances, it is not new tech, but it is very cool!

author: TT_Vert
author_rank: Regular Contributor
timestamp: March 12, 2021, 05:12:20 pm
content: At what point would that trip the breaker though? Isn't that a direct short to ground for the live wire?Dave

author: SilverSolder
author_rank: Super Contributor
timestamp: March 12, 2021, 10:01:57 pm
content: Quote from: TT_Vert on March 12, 2021, 05:12:20 pmIt is not a "short" to ground, the water has high resistance - so it is more like a resistor to ground, if you see what I mean. The leakage current is low, a regular breaker won't even notice it (they are designed to trigger on overload conditions only).Small leaks to ground is what a GFCI relay is designed to catch: it measures the difference between current out and current back (on the Phase and Neutral wires) and, if there is a difference, it means something is leaking to ground (which then makes it trip).

author: TT_Vert
author_rank: Regular Contributor
timestamp: March 15, 2021, 03:54:04 am
content: So a hypothetical here. If this motor was not grounded and IF the windings were shorted to the case I would have been electrocuted? How did they use submerged pumps back in the day before grounds? I mean there are water cooled sump motors and i cannot see the windings not ever coming into contact with that water which contacts the case.Dave

author: james_s
author_rank: Super Contributor
timestamp: March 15, 2021, 04:25:24 am
content: You would have gotten a shock if you touched the housing of the pump while it was powered and your body was in contact with a grounded surface somewhere. To get a shock you need to become a path to ground.Un-grounded pumps were not killing people left and right because people typically had the sense not to touch energized appliances that were sitting in water, and also it takes a specific set of fault conditions in order for the pump housing to become energized. A pump that is in proper working order does not need to ground in order to not give you a shock. The ground is a safety feature that is there just in case the pump develops a fault, that is not something that should be happening very often. Pure water is actually quite a good insulator, it is when you get ions in it that it becomes conductive, a bit of salt for example. Pure water though is sometimes used to cool lamp pumped lasers, the lamps including their electrical connections are submerged in the cooling water, but these require de-ionizing equipment in the cooling loop.

author: SilverSolder
author_rank: Super Contributor
timestamp: March 15, 2021, 11:50:24 am
content: Quote from: TT_Vert on March 15, 2021, 03:54:04 amYou could have got a shock in those circumstances, yes (not necessarily a lethal shock, but it is absolutely possible if you were maximally unlucky). Using grounded appliances is a safety improvement - like seat belts, it only "does something" if there is a fault condition.