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If you have used "ice cube" type PCB mount power relays before, you're probably familiar with the de facto standard pinout for them:

PCB mount power relay pinout

Why is this such a standard pinout though? It's clearly not an optimal pinout -- positioning the common terminal between the coil terminals forces you to put an isolation slot in the PCB in order to obtain reasonable creepage distances, and severely constrains the clearance distance available as well. It's also not something unique to cheap Cheese-shop-specials either: the Omron G5LE series uses this pinout, and so do equivalent relays from TE/P&B (ORWH) and Panasonic (JS1). You need to go to much costlier parts such as a Tyco PCH or RZ or Panasonic JW1 in order to get something that puts the common pin on the same side of the relay as the other contact pins.

Is there some sort of internal construction detailing that makes this type of relay unsuitable for mains isolation to begin with? Or why can't the relay manufacturers bring the common terminal out on the "correct" side of the relay to allow an isolation barrier to be established?

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  • \$\begingroup\$ Coil voltage goes to max 48V (Panasonic JS serie), contact max voltage = 250V. Is the distance in footage too small? \$\endgroup\$ – smajli Dec 31 '17 at 18:35
  • \$\begingroup\$ Because the "designers" are cost driven morons* who don't care about safety, fitness for use or common sense, and the module "designers" who use them are similarly motivated | * Formally: Moron = IQ of 50-75 in area of concern | [Idiot = 0-25, imbecile = 25-50] \$\endgroup\$ – Russell McMahon Dec 31 '17 at 23:29
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It's only to make the relay cheap. It's sub-optimal internally (electrically) as well as externally (pinout). The coil-to-contact breakdown voltage and capacitance is inferior to that of better relays. On the other hand it's cheap and the relay can be made reasonably sensitive (360mW typically for that construction).

One of the disadvantages of this construction is that the contact current always flows through the flexure, so a large surge (say to blow a fuse or circuit breaker) can anneal the spring and affect the operation permanently.

Below are a couple of photos of one I did a tear-down analysis on a few years back.

enter image description here

enter image description here

The common pin is naturally located at the other end from the contacts. They could have moved the coil pins to the front or back but either way they are close to the contact potentials unless the relay is made wider.

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    \$\begingroup\$ Cool stuff with the teardown photos! \$\endgroup\$ – ThreePhaseEel Dec 31 '17 at 22:12
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form C has a very good reason for this layout. Spoiler alert.
Sorry no answer here but good history on other aspects.
http://www.esterline.com/powersystems/ProductSupport/DesignReference/RelayHandbook.aspx

The 1 Form C defines the SPDT contact arrangement with “Form C” meaning Break Before Make in the transition. For this reason, the moving pole is wired as far away from the contact throws as far as possible to mitigate short circuit currents to avoid a Make before Break in an application where this could cause a fire from followon current if the Throws were bridged. e.g. Vbat and 0V.

Although line voltage surface creepage can occur on dusty moist surfaces and breakdown threshold may be improved with a 1 ~ 2 mm slot air gap between coil and grid connected contacts,

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    \$\begingroup\$ Could you summarize what the link has to say here in case it breaks? \$\endgroup\$ – ThreePhaseEel Dec 31 '17 at 19:15
  • \$\begingroup\$ A nice explanation for why this is desirable in some cases. || I know you know the following: Unfortunately, that is less important in the typical amateur use cases that the astoundingly low cost Asian relay modules are liable to be used for. The modules appear suitable for switching mains when driven by eg Arduinos & similar. The low cost relays used are liable to fail at rated current with less than perfect resistive loads and are also liable to be used at above rated current by undiscerning users. Failures run the risk of applying mains voltages to the low voltage circuitry. \$\endgroup\$ – Russell McMahon Jan 4 '18 at 21:42
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I agree with you, that pinout is horrible.

It likely has to do with the way the moving arm is hinged inside the relay. The hinge being on the left side of your image directly above the common pin. As such, wiring the pin on the other side would need some sort of additional internal connection. The latter does of course just move the isolation issues inside the relay.

I cant seem to find a picture of one opened up though.

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    \$\begingroup\$ I personally hate these pinouts so much. I try to select a part which has proper distance between ac and dc sides. G2RL and G5RL are two relays which I like for this very reason. \$\endgroup\$ – Whiskeyjack Dec 31 '17 at 18:41
  • \$\begingroup\$ The isolation distances inside the relay could be handled by different but still low cost construction. Those outside the relay cannot be addressed by users. Inside, the spring base does not have to penetrate the base mechanically. Some form of vertical channel could provide support, and intercontact surface distances can be made as long as sensibly desired. \$\endgroup\$ – Russell McMahon Jan 1 '18 at 1:20
  • \$\begingroup\$ @RussellMcMahon yes indeed, and better relays do, but cheap is the name of the game with these. They make it your problem not theirs. \$\endgroup\$ – Trevor_G Jan 1 '18 at 3:52

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