Weird Electrical Experience

Project_Mayhem · May 25, 2023

Most non-grounded devices, especially passive or traditional transformer/rectifier power supplies, aren't affected by reversed polarity. In my eyes, the device sees the polarity reversing 60 times a second. I'm well aware that it's "referenced to ground/neutral".

It is possible that the modem has some circuitry that detects reversed hot/neutral to avoid sending voltage down the coax's shielding. It really should be electrically isolated but nobody really knows without analyzing the circuit design.

I'd take a look at the prong thickness as something too thin may cause a bad or no connection. It's not likely but possible

Ground differentials are what usually cause people to get shocked. I sometimes setup equipment in an industrial building that has circuits coming from from two separate breaker boxes. While plugging in a signal cable from equipment plugged into the other box, I felt a decent shock. I was handling the metal housing of the cable. I suppose it's also possible that something was miswired and one circuit was on a different phase. The building is over 100 years old so many people have worked on the electrical system over the years

walt · May 25, 2023

Possibly.. Having a reverser polarity detect to power up makes more sense with equipment that can have a signal cable go to another box plugged into a separate socket. You may not have a cable between boxes but the router might have for example ethernet cable ports for some option.

Breaker box's are typically wired so that 120V breakers alternate phases horizontally. So a breaker powering one room might be right next to a breaker powering the next room but with the opposite phase. No problem at all connecting equipment powered from the different phases if the polarity is correct. But if one of the sockets has polarity backwards, you could have 120V between the cable grounds.

RoyS · May 26, 2023

walt said:
Breaker box's are typically wired so that 120V breakers alternate phases horizontally. So a breaker powering one room might be right next to a breaker powering the next room but with the opposite phase. No problem at all connecting equipment powered from the different phases if the polarity is correct. But if one of the sockets has polarity backwards, you could have 120V between the cable grounds.

Not so. Assuming system grounding is correct, you cannot get a shock from cable armor. Cable armor and all metal boxes in the system are bonded to the grounded neutral wire at the breaker panel and further connected to the service water pipe and, lately, to several driven ground rods. The reason for all of that is to ensure that should a short with a hot wire coming in contact with any metal surrounding the wiring, the short will be carried to ground and immediately trip the breaker or fuse. Polarity reversal at a socket, while incorrect, will not cause the cable armor to become hot and electrocute you. In older homes, however, there are often homeowner wiring errors that lose the original grounding protection. With still older ungrounded systems and particularly with knob and tube wiring mixed with bx, all bets are off.

walt · May 26, 2023

yep, just a guess. The device does appear to have a polarity sense that prevents power up and likely has cable ports (actual cable coax plus Ethernet ports).

I am not sure of the correct way to describe things because standard 240 coming into a home from a transformer is single phase. But the 240 is three wire (and ground) and you get two 120 VAC lines with respect to the nuetral and each of those two 120VAC lines are 180 degree out of phase from each other. The breaker box 120 VAC breakers alternate on which phase is used horizontally. So you can have 120 VAC sockets in one room that are 180 degree out of phase with the 120 VAC sockets in the next room. The neutral's are common but if you were to connect the 120 VAC hot wires.. you just connected together the 240 VAC wires.

If the box design has the cable shields all tied to ground as reference, the wrong polority plug would not matter. But if the cable shields were somehow tied to nuetral, you could have a problem with the wrong polarity. I actually was involved in satellite set top box design thousands of years ago and cant remember grounding details. All I do remember is that the metal chassis was always tied to the earth safety ground. There would been a switching power supply that provided all the DC voltages (and signal ground) but I dont know if or how these would have been tied back to AC neutral.

Could be wrong.. just a guess. FYI, I m not sure when this happened but almost any newer transformer will put out 120/ 240 because there is less power loss in wires. My home actually has 246 VAC comming in.

Project_Mayhem · May 26, 2023

walt said:
Possibly.. Having a reverser polarity detect to power up makes more sense with equipment that can have a signal cable go to another box plugged into a separate socket. You may not have a cable between boxes but the router might have for example ethernet cable ports for some option.

Breaker box's are typically wired so that 120V breakers alternate phases horizontally. So a breaker powering one room might be right next to a breaker powering the next room but with the opposite phase. No problem at all connecting equipment powered from the different phases if the polarity is correct. But if one of the sockets has polarity backwards, you could have 120V between the cable grounds.

Issues can occur when running signals between devices on two different phases. I ran into this when running cabling to TV's in a bar. I know I've run into it at other places but that was the most predominant

Crusty Ol Salt · May 27, 2023

Stu Jackson said:
Remember the old days when it didn't matter which direction you plugged a two prong A.C. plug into a receptacle? Before the ground plug was added and before the larger prong was introduced?

Wait, what???

How could I never noticed this.

Crusty Ol Salt · May 27, 2023

jviss said:
Yes, non-polarized, two-prong plugs. Not very safe.

I am amazed that you can still find polarized but ungrounded appliances.

Project_Mayhem · May 30, 2023

walt said:
yep, just a guess. The device does appear to have a polarity sense that prevents power up and likely has cable ports (actual cable coax plus Ethernet ports).

I am not sure of the correct way to describe things because standard 240 coming into a home from a transformer is single phase. But the 240 is three wire (and ground) and you get two 120 VAC lines with respect to the nuetral and each of those two 120VAC lines are 180 degree out of phase from each other. The breaker box 120 VAC breakers alternate on which phase is used horizontally. So you can have 120 VAC sockets in one room that are 180 degree out of phase with the 120 VAC sockets in the next room. The neutral's are common but if you were to connect the 120 VAC hot wires.. you just connected together the 240 VAC wires.

If the box design has the cable shields all tied to ground as reference, the wrong polority plug would not matter. But if the cable shields were somehow tied to nuetral, you could have a problem with the wrong polarity. I actually was involved in satellite set top box design thousands of years ago and cant remember grounding details. All I do remember is that the metal chassis was always tied to the earth safety ground. There would been a switching power supply that provided all the DC voltages (and signal ground) but I dont know if or how these would have been tied back to AC neutral.

Could be wrong.. just a guess. FYI, I m not sure when this happened but almost any newer transformer will put out 120/ 240 because there is less power loss in wires. My home actually has 246 VAC comming in.