Aquarium and electrical safety

Euphylliaphyle · Mar 1, 2026

Paul B said:
They were these types of knife switches but weighed a couple of hundred pounds.

Utwo229 · Mar 2, 2026

BeanAnimal said:
I am sorry, but that is not even remotely correct.

MOVs are "Metal Oxide Varistors" they are surge clamping devices and have absolutely nothing to do with the function of a circuit breaker or a GFCI.

The circuit breaker in a residential panel is a thermo-magnetic device. There are two trip mechanisms.
The first is a solenoid. It is set to physically push on the trip mechanism at very high overcurrent levels that are usually many times higher than the breaker's nominal rating. This is the "instant" trip that prevents a massive short from melting the branch circuit wiring. This can be found on the time current curve for the breaker as the vertical line near the 0 second mark.

The second is a bi-metal strip that distorts under thermal load and forces the trip mechanism to activate at a preset current level near the the breaker’s nominal rating. This is the meat of the sloped portion of time current trip curve. This is the part that may weaken over time and alter the breaker’s behavior.

A GFCI uses a differential current transformer and a solid state comparator (a chip) to measure the delta in current between what flows in on the hot leg and out on the neutral. If it detects an imbalance of more than ~5mA it uses the circuit’s power to energize a solenoid against the trip mechanism.

MOVs are used in surge protection. They are normally very high resistance devices connected between conductors of a circuit. Repeated surges lower the clamping voltage and they eventually fail. Yes, large surges or lightning strikes can destroy them as "sacrificial" protectors. In high quality surge protectors, we parallel MOVs with avalanche diodes. They offer some protection to the MOVs by clamping fast transient surges before the MOVs have to handle them.

There may be some GFCI receptacles or various types of breakers with "surge protection" built in, and that surge protection may be MOVs and/or avalanche diodes, but this is not common and has NOTHING to do with the actual function of the breaker or the GFCI. It is there to offer surge protection for the circuit.

Not looking to start a fight with you on this, but your first comment is unnecessary and in fact you probably misunderstood my reply. I was just trying to point out that GFCI's do sometimes wear out. And I am not saying that a GFCI device is a MOV, but only that there are internal MOV components on the AC input side of GCFI devices to protect the sensitive electronics from periodic surges. And over time, if susceptible to many surges (like we have in Florida, the lightening capital of the world) the MOV component degrades and may cause the GFCI to become unstable.

Additionally, I have experienced an and electrician, that GFCI devices that are subject to many and frequent tripping can become weakened and more suspectable to nuisance tripping. They do wear out. This is just one reason GFCI's have test buttons and the manufacturers suggest they be tested regularly.

In fact, two significant changes in UL 943 (effective June 29, 2015) required GFCI devices to all have self-test logic and if they fail, to disable the device from being reset. Prior to that update, one vendor had a patent to fail without allowing power to be restored if the test button triggered the failure. All the other vendors just allowed the device to be reset and provide non-GFCI protected power even when the test button identified a failure.

So even though all post June 2015 GFCI devices will stop providing power when they fail either from self-test or manual test button, I can guarantee that many people still have prior version GFCI's installed and could think they have GFCI protection when in-fact they do not.

BeanAnimal · Mar 2, 2026

You originally stated: “GFCI breakers and receptacles use a MOV to detect the trip point.”
That is incorrect and prompted my response.

A GFCI detects imbalance using a differential current transformer and comparator circuit. The trip point is defined by the sensing transformer and electronics, not by a MOV, likewise (see below) MOVs are not typically used to protect internal GFCI components.

I responded to make those points clear.

Standard GFCIs do not include internal MOVs for any reason. Some may include small transient suppression components, usually RC snubbers or TVS diodes for internal protection, but not MOVs.

I can explain why from an engineering standpoint if you have interest.

The UL 943 2015 revision requiring periodic self-test and end-of-life lockout has been cited and covered in these threads numerous times and the standing advice has been for people to have an electrician check for and replace outdated devices. No issue with you pointing that out, it is correct.