What stops the power 'jumping' between the pins of jst connectors and shorting? - Page 1
Forum section: electronics / beginners

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: February 26, 2025, 08:15:46 am
content: I have bought a few of these:https://www.switchelectronics.co.uk/products/2-way-male-prewired-jst-smp-connector-15cm?_pos=10&_sid=aecc4e2c7&_ss=rhttps://www.switchelectronics.co.uk/products/2-way-female-prewired-jst-smp-connector-15cm?_pos=16&_sid=aecc4e2c7&_ss=rI got them with the idea that rather than having to make a new fuse block wire run every time for circuits of similar current I could just swap the connector for different devices.I got to wondering though, since when I get + and - wires to touch accidentally this often blows a fuse. When the male is not connected, these exposed pins are rather close together so what stops the current from 'jumping' and doing the same thing?

author: wasedadoc
author_rank: Super Contributor
timestamp: February 26, 2025, 09:44:15 am
content: Until the voltage difference is high enough (volts/metre) to ionise it, air is an insulator.

author: Psi
author_rank: Super Contributor
timestamp: February 26, 2025, 09:54:19 am
content: Quote from: wasedadoc on February 26, 2025, 09:44:15 amYep, or if the pressure gets super low and you have some unusual gas mixes around, then things can get really crazy.Distances that you'd never think would arc over in a millions years suddenly do at very low voltages.Things like 900V jumping a meter.

author: brucehoult
author_rank: Super Contributor
timestamp: February 26, 2025, 10:02:32 am
content: Because you need about 3000 V to ionise a 1mm gap between parallel plates or wires, making a spark.However it might be as low as 1000 V per mm for pointy tips, so better to keep below that anyway.The connector you point to has a lot more gap than that, and will be used with a lot less voltage.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 26, 2025, 08:15:26 pm
content: If you look at thedatasheetyou'll see that the JST SM series is rated for up to 250V. This is theoperatingvoltage, the level at which it is guaranteed to perform as advertised. So you can see that even a fairly small connector like this can be used at quite high voltages. Note though that there is another voltage rating, thewithstanding voltage, which is 1500V AC. This is the level at which the insulation (i.e. the plastic) between pins may break down and allow current to pass - even a very tiny amount. So bear in mind it's not just about the air between the pins, but also the plastic.(BTW, why in the datasheet is the withstand voltage labelled just as "1500V AC/minute". I thought that V/minute was to do with ramp testing, but specifying the ramp rate without a maximum doesn't make sense to me. I'm sure they mean a maximum of 1500V, so are they also implying "ramp to 1500V over a period of 1 minute"?)

author: SteveThackery
author_rank: Super Contributor
timestamp: February 26, 2025, 09:07:27 pm
content: Quote from: HwAoRrDk on February 26, 2025, 08:15:26 pmI'm guessing it means it'll withstand 1500V for one minute.

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2025, 09:59:47 pm
content: Quote from: brucehoult on February 26, 2025, 10:02:32 amIt might be less than 1000V/mm on a humid day. 3000V/mm is for dry air. If there is any absorbent contamination on the surface of the plastic a humid day might see significant leakage even for low voltages. You can't just quote people best case conditions.

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2025, 10:02:35 pm
content: Quote from: HwAoRrDk on February 26, 2025, 08:15:26 pmThere are lots of test specs that equipment have to pass which have that 1500V requirement. Its actually 1500V sustained for one minute. You'll find it in lots of land line telecoms specifications, for example.

author: HwAoRrDk
author_rank: Super Contributor
timestamp: February 26, 2025, 10:19:03 pm
content: I see. So why write it as 1500Vperminute? To me that implies a rate of change.

author: coppice
author_rank: Super Contributor
timestamp: February 26, 2025, 10:27:10 pm
content: Quote from: HwAoRrDk on February 26, 2025, 10:19:03 pmIts a shorthand you'll see on many things. If you see 1500V/min where it can be interpreted as "passes the 1500V for one minute insulation breakdown test" it probably should.

author: RJSV
author_rank: Super Contributor
timestamp: February 26, 2025, 10:43:59 pm
content: The volts per meter figure is for CLEAN areas near the pins, so that's a factor that needs to be happening, ongoing.Long power cords introduce some degree of inductive kick-back, especially with big loads and motors.00Manufactures sure do push limits on things like connector size, which is unfortunate but consumer pressure can be daunting, (to go smaller).

author: brucehoult
author_rank: Super Contributor
timestamp: February 26, 2025, 10:56:49 pm
content: Quote from: coppice on February 26, 2025, 09:59:47 pmSure, you should leave a margin. I'm just pointing out that you should leave a margin from 1000V/mm, not from 3000V/mm.250V with a few mm gap between is not going to be a problem in any situation bar contamination causing a short by something other than air.

author: electroniclearner820327
author_rank: Regular Contributor
timestamp: February 27, 2025, 08:46:01 am
content: Ok well I only plan to use on 12v system so should be no issues in my case.

author: PGPG
author_rank: Frequent Contributor
timestamp: February 27, 2025, 09:41:36 am
content: Quote from: HwAoRrDk on February 26, 2025, 10:19:03 pmMinute for electronic is eternity.If it were 1500V/us you could expect it is rate of change specification but not when the time is minute.Some tests are specified to make test for eternity (=1 minute) and you should it read as device passes that test.

author: brucehoult
author_rank: Super Contributor
timestamp: February 27, 2025, 10:00:30 am
content: Quote from: electroniclearner820327 on February 27, 2025, 08:46:01 amWith 12V on something designed for 240V you are more likely to run into current limitations!

author: CaptDon
author_rank: Super Contributor
timestamp: February 27, 2025, 01:30:30 pm
content: What stops it is the same thing that stops the electrons in the 240v wall socket from frying you as you sit in your recliner 2 metres from the socket. Air makes a real good insulator 99.9% of the time.

author: golden_labels
author_rank: Super Contributor
timestamp: February 27, 2025, 02:49:44 pm
content: As a supplementary read, if you want to learn more on the topic, the three keywords you need to search for:ClearanceCreepage (or creepage distance)DIelectric strengthAtable of dielectric strengthson Wikipedia gives a rought feel of what values we’re talking about. Note that locally field strength depends on conductor shape. As somebody mentioned earlier, electric field will have very high value around sharp edges/points, easily exceeding the limit. We need to take that into account and not blindly assume that 5 mm gap can unconditionally withstand 15 kV in air. Otherwise you could just put on a pair of work gloves and hang from the nearby high tension line (which I strongly suggest not to do).To all what has been written above, there is a small asterisk. It holds very well for the original scenario of the connector, a spark, and a fuse blowing. No worries about that! But it only remains true around the DC regime. I don’t know, how much did you learn so far. But don’t get too much attached to the notion of insulators not conducting current. Not in unconditional, absolute sense. Dielectric strength decreases with rising frequency, and also DC resistance gets replaced with frequency-dependentimpedance. The former will not be a practical problem unless you design chips or go into materials science, the latter will have no implications for electrical safety. But worth knowing this is only a simplification.