# Questions about home electrical wiring, hallway light switches and latching relays **URL:** \ **Category:** Factual Questions\ **Created:** [January 3, 2023, 5:07pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596 "2023-01-03T17:07:18Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 5:07pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/1 "2023-01-03T17:07:18Z")
I have two hallways in my home. Each of them has a light operated by three remote switches and a latching relay/impulse switch. Here is a picture of the type of relay they use: [https://ibb.co/wK2qhmd](https://ibb.co/wK2qhmd) Here is a picture of the junction box in one of the hallways with the relay installed. [https://ibb.co/60XRHx6](https://ibb.co/60XRHx6) Here is a picture of a typical switch. [https://ibb.co/0j9Hf2j](https://ibb.co/0j9Hf2j) Question 1: Does the following picture accurate represent my set up? [https://ibb.co/NKhQPV1](https://ibb.co/NKhQPV1) Question 2: Is the relay in this picture a functional replacement for the one shown above? Will it serve the same purpose if a replacement Siemens 5TT5 031 can’t be found? [https://ibb.co/44syYz7](https://ibb.co/44syYz7) Question 3: Here’s the part that I’m having trouble wrapping my head around. See the switch labelled in the schematic above. I’ve labelled each side of the switch as 3A and 3B. Using a multimeter with one probe on 3A and another on 3B, it reads 230 volts. This is true regardless of the position of the relay switch (that part makes sense), but also regardless of whether the light is turned on or not. I’m sure I am missing something obvious, but how can there be a complete circuit to read 230 volts, if the light is off? Please help me understand this. Yes, I know I should hire an electrician. In true SDMB fashion, I am going to ignore all sane advice to do so, but I really appreciate the help and thank you all in advance for sharing your knowledge with me! ---
**Author:** ![Seanette](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/seanette/32/9139_2.png) [@Seanette](https://boards.straightdope.com/u/Seanette)\ **Post date:** [January 3, 2023, 5:25pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/2 "2023-01-03T17:25:44Z")
My husband is an electrician. I’ve called this thread to his attention, but he’s at work right now, so it’ll probably be this evening our time. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 5:47pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/3 "2023-01-03T17:47:23Z")
> [@Seanette](#): > > I’ve called this thread to his attention, but he’s at work right now, so it’ll probably be this evening our time. Awesome! I’m looking forward to hearing from him. Thanks! ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 3, 2023, 8:35pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/4 "2023-01-03T20:35:16Z")
Your circuit looks correct, but your measurements must be wrong. As drawn, there should be ZERO volts between 3A and 3B when the light is ON, and 230V when it’s OFF. So, something is not right… ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 8:57pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/5 "2023-01-03T20:57:46Z")
> [@beowulff](#): > > Your circuit looks correct, but your measurements must be wrong. > As drawn, there should be ZERO volts between 3A and 3B when the light is ON, and 230V when it’s OFF. … deleting my previous comment because I misunderstood something. Just want this deleted while I rewrite a response. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 9:01pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/6 "2023-01-03T21:01:47Z")
> [@beowulff](#): > > As drawn, there should be ZERO volts between 3A and 3B when the light is ON, and 230V when it’s OFF. ZERO when ON but 230 when off?? I am pretty sure I only took a reading when it was OFF, never on. I’ll be right back. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 9:06pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/7 "2023-01-03T21:06:57Z")
I just checked, and this is the case. It is 230 when OFF, but 0 when ON. I might be even more confused. This is most definitely the concept that I’m missing. Can you help me understand why this is the case? I feel like it should be the opposite of that because a light turned on would mean the circuit is complete, but if it is off, there is a break in the circuit, so there should be zero volts. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 3, 2023, 9:24pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/8 "2023-01-03T21:24:39Z")
Ohms law, son. Ohm’s law. V = IR. The voltage across two points is equal to the current passing between them multiplied by the resistance. In this case, the resistance is zero ohms (or near as damn) when the contacts are closed. So V = (current through the bulb) \* zero = zero. It’s a little less clear when the contacts are open. Then the resistance is infinite, and so one would think that the current would be zero, hence the voltage is undefined. But, what really happens is the meter has finite resistance, so some tiny amount of current flows through it. It’s so small the any other resistances in the circuit are irrelevant, so the measured value is the supply voltage. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 3, 2023, 9:32pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/9 "2023-01-03T21:32:33Z")
As for the relay - the replacement you proposed looks correct, but I couldn’t find an English data sheet, so I’m not 100% confident. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 9:39pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/10 "2023-01-03T21:39:40Z")
> [@beowulff](#): > > so some tiny amount of current flows through it. Flows through the meter, but doesn’t need to return through the N wire back to the supply source to “flow”? Then would touching the supply side at 3A and just any other random wire not on the circuit also produce a 230 reading? Because the wire at 3B is just a wire segment that isn’t connected to N at all when no bulb is present. And it still reads 230 even with the bulb removed, so there isn’t anything there to complete the circuit. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 9:41pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/11 "2023-01-03T21:41:55Z")
> [@beowulff](#): > > As for the relay - the replacement you proposed looks correct, but I couldn’t find an English data sheet, so I’m not 100% confident. Thanks! This has been a huge help so far. I really appreciate it. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 3, 2023, 9:44pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/12 "2023-01-03T21:44:09Z")
> And it still reads 230 even with the bulb removed, so there isn’t anything there to complete the circuit. This shouldn’t happen. There should be no current at all with the bulb removed, so the voltage should be zero. But, didn’t you say that there were two bulbs? Do they both come on at the same time? If so, remove both and see. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 9:51pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/13 "2023-01-03T21:51:24Z")
> [@beowulff](#): > > This shouldn’t happen. Okay. Perfect. While I was typing my last response, I started to mention about not having a complete circuit to N with the light turned off, but then I realize the filament is all still there, so there is a circuit, there just isn’t enough flow through the meter to turn it on. I realized that while talking through it (typing through it). But then I realized the light downstairs has the bulb removed but I’m still getting the same reading. My conclusion there is that either I’m reading the wrong wires or the filament isn’t necessary. Reading your response here, I think I’ve got that answer. Okay, so the upstairs (the hallway with two lights and a functioning relay) has its bulb, and I clearly understand now why the readings are what they are. The downstairs hallway has only one light. That bulb is currently removed, and so is the relay. I was just reading the bare wires sticking out of the junction box. I need to go check that whole thing again. As I type this, I’m thinking I might actually have a bulb in it–or at least, I did when I took the readings. Back in a sec. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 10:05pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/14 "2023-01-03T22:05:08Z")
Booyah! That was the key! Went down there and the bulb was out. No reading. Put the bulb in and 230. So, I must have just taken the bulb out downstairs at some point “after” taking the readings, not “before” as I had thought. Later while trying to understand the measurements I was getting and how the whole circuit worked, I just misremembered and thought I removed it earlier. I just never thought about it or associated it to playing a part in anything, and then just thought it had been out the whole time I was taking the readings. Until a few minutes ago, it didn’t occur to me that the presence of a light bulb was completing the circuit during the reading, regardless of whether it was on or not. It’s so obvious now, but I never would have got there without your help. I knew there was some common concept I was missing and that was it. Thank you! This has been racking my brain since Sunday, but it seems comically simple now. Thanks again. ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 3, 2023, 10:09pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/15 "2023-01-03T22:09:53Z")
The replacement relay will be here tomorrow. I’ll let you know how it goes. I’m excited to get this fixed. Hell, Sunday morning I had no idea what a latching relay was or why some of my light switches are springy buttons and others regular switches. So I’ve come a long way in two days, I think. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [January 3, 2023, 10:11pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/16 "2023-01-03T22:11:33Z")
Fingers crossed. ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [January 4, 2023, 11:04pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/17 "2023-01-04T23:04:26Z")
Yikes, I’m a sparky in Wisconsin and none of this stuff looks like anything I’ve installed in a home before. Is this a prefab? Are you in the States? ---
**Author:** ![Bear\_Nenno](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bear_nenno/32/3358_2.png) [@Bear\_Nenno](https://boards.straightdope.com/u/Bear_Nenno)\ **Post date:** [January 4, 2023, 11:35pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/18 "2023-01-04T23:35:03Z")
Germany. Not a lot of lumber used in home construction here. Walls are like a thick, solid sandstone rather than studs and drywall. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [January 5, 2023, 2:11am UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/19 "2023-01-05T02:11:56Z")
Welcome back @Uncommon_Sense. We (or at least I) had missed your cool moniker & great advice. ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [January 5, 2023, 8:29pm UTC](https://boards.straightdope.com/t/questions-about-home-electrical-wiring-hallway-light-switches-and-latching-relays/977596/20 "2023-01-05T20:29:52Z")
Thanks LSL! Been away for several years.