Green(Ground) and black (Negative) are shorted on my PSU, is that normal?

[AmeliaBuns]

Hi, I was trying to do something and I noticed a huge spark as my Bench PSU's red terminal touched my scope's ground lead, there was a huge spark, I'm hoping my scope is ok, everything is measuring just fine but i'm really worried! It was strange how bad it was as my PSU was set to 0.1A current limit.
the power supply was set to 24v.

however, I was under the impression that most bench power supplies with 3 banana plug terminals are isolated? I took my multimeter and noticed that the Black and green randomly get shorted every few seconds (since i used my multi meter and don't know how to use a scope for continuity testing, i can only say that my multimeter kept switching between open and a very low resistance?!

is this behvaiour normal?  Why would it be switching ? I thought most bench power supply outputs are in a way, isolated and not connected to the actual ground.

also i'm new to electronics so please bear with me! 

[bdunham7]

What model PSU do you have?  On most linear bench PSUs the outputs are indeed isolated, although this can vary with the number of channels and so forth. 

[KungFuJosh]

I hope you're not testing continuity on a live transformer. You should be using resistance if it's powered, and be very careful using properly rated probes.

[jjoonathan]

There is a hidden trap for new players in the "social contract" of most DC power supplies: the current limit applies before the output capacitor. This means if you set your power supply to 10V 1mA and connect an LED directly to the output (or connect both outputs to ground), the power supply will not pump more than 1mA into its output capacitor but the output capacitor will happily dump far more than 1mA into the LED until its voltage has fallen to steady state. This kills the LED (or causes a spark) because the short circuit current of the capacitor is large. Intro EE classes often buy fresh LEDs for their first lab class because it is expected that many students will learn the hard way, even if the instructions try to warn. You are not alone in this, it's a universal experience.

I have noticed extra caveats that come with cheap power supplies. A common one is that if you do not turn off their output before cutting power, they dump their intermediate voltage caps directly into the output (in my case, this was 42V for about 20ms). I have also seen a protection function that when engaged does not go to zero but rather outputs -1.2V and -20mA on a supply that was supposed to be positive only. The bigger point is: cheap power supplies do weird things. I have not seen the behavior of periodically shorting the negative terminal to ground, I would typically expect the terminal to be isolated, but I would not be surprised to see such silly behavior from a cheap supply.

As you might expect, more expensive supplies tend to diverge from baseline in the opposite direction. Expensive power supplies from Keysight (or so) will sometimes support "downprogramming" -- intentionally draining their own output capacitor to reduce voltage quickly for waveform generation or to quickly enforce current limits. SMUs also typically don't have the output capacitor caveat. If you set them to 10V 1mA and short the output, the only surge will be the surge required to drain the parasitic capacitance of the wires. The 1mA limit will apply inside a microsecond rather than after many milliseconds. You pay a pretty penny for these features, though, so it is best to learn how to deal with regular power supplies first.

The oscilloscope is probably fine. They are usually constructed with probe ground directly connected to the chassis. You could likely dump enough current into your probe ground to make the wires glow red hot without damaging the actual signal processing hardware in the scope. The melted probe would need to be replaced, of course, but that would be the end of it.

[IAmBack]

Positive and negative terminals should be isolated from green "ground" terminal, but ground terminal is connected to ground pin in AC socket. Also BNC shield in your oscilloscope is connected to the same AC ground. If in Your supply negative terminal is shorted to ground, it may cause severe damages in Your circuit while measured with oscilloscope.

[Kean]

The oscilloscope is probably fine. They are usually constructed with probe ground directly connected to the chassis. You could likely dump enough current into your probe ground to make the wires glow red hot without damaging the actual signal processing hardware in the scope. The melted probe would need to be replaced, of course, but that would be the end of it.

There was actually a recall on the Tek TDS210/220 because of this.  The PCB trace from the BNCs to chassis earth was too thin and could actually go open circuit and cause a safety issue.
https://www.tek.com/en/service/safety/tds210-tds220

Back to the OPs questions...


What jjoonathan mentions is the cause of the spark - the output capacitors will discharge rapidly despite the current limit, especially at something like 24V.

Was there some other connection from the scope input ground leads to your power supply with some intermediate circuitry?

Not sure if we can explain the oscillations of the continuity between the black and green terminals without more info.  I'd guess that it is open circuit, and your DMM is picking up stray noise through the PSU.  You should try it again with the power unplugged and report back.