Do I still need a fuse if I'm using a GFCI?

Question

RE: PD involving a small health & beauty product similar to a foot spa whereby the user will come into contact with water.

General description: Because malfunctioning foot spas have been known to kill, we have chosen to use a silicone heat pad (IP65) mounted under the stainless steel bottom so that the heater will never come into contact with the water unlike a foot spa which uses a coil heating element (similar to a household water heater) which has direct contact with the water and a pump which circulates the water. Also, we will use a quality 14/3 main power cord and a GFCI plug similar to this one but less expensive.

Two questions:

1. Since we are using a GFCI, do we still have need for the 15amp fuse at the main power inlet or on the PCBA, or does the GFCI take the place of the fuse? By my limited knowledge (see my notes below,) it seems like a GFCI can take the place of a fuse but not the other way around.

Note: My knowledge of the differences between the two: GFCI - if the current flowing into the circuit differs by a very small amount (typically <.006 amperes) from the returning current, the GFCI interrupts power to prevent a lethal dose of electricity. Fuse - if more electricity flows through a fuse than it was designed for, the fuse heats up to a point of melting which stops the flow of electricity because of an open gap in the circuit thereby protecting the components.

2. If it is determined that a 15A main power fuse is still beneficial in spite of using a GFCI, is it better to have it located at the main power inlet or as a component of the PCBA? Note: remember, this is for mass-production so ease of product assembly and raw material as well as labor costs are always a significant factor.

Once I complete the general design, I will turn it over to a team of engineers who will take it to the next level. Although it doesn't need to be perfect when they get it from me, I desire to look competent in this regard which is why I'm here searching for answers.

Edit 6/10/2023 See link to follow-up question here: Circuit Protection: on-board vs off-board; manual vs auto. Which is better for our application?

Answer 1

GFCI takes care of current going someplace it shouldn't, which includes into/through people. That is vital for anything with water, and you really need to supply it as part of the appliance. It has to be at the end of the cord because that way it protects every scenario, including wet hands on a damaged cord. Even though the NEC may well require GFCI protection on all receptacles in areas where you expect your product to be used, that is not enough because older locations are normally grandfathered and so you (the device manufacturer) can't guarantee the user will use a GFCI-protected receptacle even if you put that in the instructions. Since (at least in the US) any 120V 15A device can be plugged into any 120V 15A (or 20A) receptacle, regardless of whether the receptacle is GFCI protected or not, you can't rely on that.

The overcurrent protection is a different story. It is designed to protect the wiring, the device and the building from overheating and fire. You can assume that any receptacle for a 120V 15A device will include either 15A or 20A overcurrent protection, as that has been the standard for about a century. Some users may come up with crazy schemes - e.g., splitters off of 30A receptacles - but that is not your problem, and in fact such splitters, if they are actually safety certified will include overcurrent protection equivalent to what a 15A or 20A circuit would normally have in a fuse or breaker panel.

So that leaves the question of whether your device needs better overcurrent protection than the standard fuse or breaker. If the device normally uses close to the full rated power of a 15A circuit (12A continuous or 15A short term) then you may decide that additional protection isn't needed, depending on likely failure modes. A 12A device could use up to ~ 25A (20A + 20%) before tripping a standard 20A breaker. If likely catastrophic failure (i.e., where you would want to stop the entire device to avoid more damage) is likely to happen at far less than 25A then having your own protection (perhaps at 15A) makes sense. On the other hand, if likely catastrophic failure would take the form of a short circuit pulling 30A or more then relying on the standard (building circuit supplied) breaker is quite reasonable.

That being said, UL, ETL or other certifying agencies probably have far more detailed rules about when you must have your own protection, and assuming you pursue such certification, those rules matter far more than random people on the internet.

In addition, often a device will have smaller fuses (or breakers) in specific locations - e.g., to protect a motor controller board if the motor it is controlling overloads. While not fundamentally different from circuit overcurrent protection, the values could be quite different - 3A on a small circuit, etc.

Answer 2

It may be useful to provide another answer, further clarifying the pros and cons of various forms of circuit protection.

I'm not certain that an auto-reset circuit breaker will provide the required protection. It may be appropriate for short duration transient overloads such as a motor, but for a major fault like a shorted component, it would be dangerous if it continues to cycle on and off. A PCB mounted fuse would probably be inexpensive, and could be used in conjunction with the auto-reset breaker.

If a component fails and causes a short or overcurrent between line and neutral, the GFCI will not trip, and the breaker will continue to cycle on and off. It looks like SMT fuses rated 125 VAC are readily available up to at least 5A, for 25 cents or so. For 250 VAC prices are more like 50 cents.

You might also consider PTC thermistor type resettable fuses, which limit fault current to a relatively safe value, and also in the 50 cent price range. As a point of reference, a 1A PTC fuse has a trip current of 2A and will trip in 15 seconds at 5A. It will then stabilize dissipating 2.64W and allows 22 mA into the load. This occurs at a temperature of about 125 C.

Datasheet: https://www.mouser.com/datasheet/2/643/ds-CP-0zrm-series-1664109.pdf

Note that this 1A PTC device has a maximum current of 10A, while its initial (cold) resistance is 0.18 to 0.47 ohms. So a solid short would result in an instantaneous current of 255 to 667 amps. This maximum current is also the interrupting rating, and that should be taken into account. A comparable 1A 250V fuse has an interrupting capacity of about 200A.

Answer 3

The #1 question here has been answered; we do need an overcurrent circuit breaker with the GFCI. However, the answer to #2 regarding the ideal circuit protection for my particular application has become way more complex than I originally anticipated. At the suggestion by @PStechPaul, I agree that it would be beneficial to close this thread and open a new thread specifically relating to circuit protection. Once I ask the new question, I'll post the new thread link here for future reference.

I hope I'm doing this correctly and not breaking any rules.