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I would appreciate help understanding the difference between an RCCB and MCB in the context of the situation I'm sharing below.

I have a mini fountain pump in my pond which doesn't have any earth pin (i.e. it has only 2 pins) and it connects directly to the electrical socket. Let's say the submerged portion of the electrical wiring somehow gets cut and exposes the copper wires to the water. An RCCB would immediately cut off the electricity. But an MCB would perhaps take a few seconds to kick in (or would it not kick in at all, given that the pump consumes low current/power?). Is my understanding correct?

Based on the past hour of reading articles and watching videos, I understand that RCCB's work by monitoring outgoing current vs incoming current. When there is a difference (domestic ones seem to kick in at 100mA), the RCCB shuts off the circuit.

But then, wouldn't an MCB do this too? Going through some data sheets, it looks like an MCB needs a much higher current, i.e. 3x the house load. So is the difference that an RCCB kicks in at a very low current (100mA) vs 18 Amps for an MCB?

and what about the reaction time - how long should the current flow for each device to kick in?

I'm in a rural place so don't really have alternates than the 2 pin (ground wire missing) mini fountain pumps. I plan to put about 3 of them into my pond. The house electrical panel already has an RCCB. Is there any reason to install one more RCCB at the electrical outlet next to the pond that's powering these pumps?

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    \$\begingroup\$ an MCB won't shut off, if only one wire touches the water, and then you stick your hand in the water and get electrocuted. \$\endgroup\$ – user253751 Jul 28 at 17:10
  • \$\begingroup\$ You've asked 6 questions now and have not formally accepted any answers. Is this something that you were not aware of? You keep fish (so do I) so, fit a proper RCD (RCCB) that trips as low as you can get. UK ones tend to trip at 30 mA and I don't rely on my house RCD - I have one on my outlet to the pond (big fish and big disappointment if they fried). \$\endgroup\$ – Andy aka Jul 28 at 17:30
  • \$\begingroup\$ @Andyaka that's right. As silly as it may sound, I was not aware of this until now. Have gone ahead and fixed that for my previous posts. Thank you for pointing it out! Definitely don't want fried fish. Let me look up RCCB options at the local store. \$\endgroup\$ – SamuraiJack Jul 28 at 17:45
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    \$\begingroup\$ When I read this title, I thought this was about how to protect undersea cables from geological faults forming. \$\endgroup\$ – DKNguyen Jul 28 at 17:58
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    \$\begingroup\$ Aside, it would be interesting to visualize where this leaking current would go through the pond water exactly. Would it look like a cloud, densest at the point source, or lightning branching out and changing, or? Everyone assumes the entirety of the water becomes "deadly" but I doubt that would happen in reality. Perhaps if it were salt water in an insulated aquarium - stepping one foot into that (and one on ground) would be "enlightening", sure. But a pond? I'd think the current would want to go downward, but not sure. \$\endgroup\$ – rdtsc Jul 28 at 18:16
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The appropriate device to protect against current leakage (i.e. appliances submerged in water exposing electric current) is RCCB. Essentially an RCCB detects any leakage currents, be it from a current carrying wire opening up under water or from a living being (person, animal or even fish) getting electrocuted due to whatever fault in the appliance.

On the other hand, an MCB protects against short circuit or current overload. And it protects the wiring.

In summary - RCCB - protects the person (or living beings) MCB - protects the wiring in the house

Special thanks to user253751 for commenting below with an example in which an RCCB would work but an MCB would not.

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