1. In the plug head
The device is moulded into the plug that goes in the wall. The entire cord and the load are protected. You reset it at the outlet. It suits extension cords and equipment leads where the outlet is reachable: job sites, workshops, kitchens, anywhere the cord is the last thing between the wall and a tool. The plug head is bulkier than a standard plug because it holds the electronics and the two buttons, so it may not fit a recessed outlet or a crowded strip. On a PDU or in a tight junction box, choose in-line instead.
On worldcordsets.com: NEMA 5-15P GFCI cords, the L6-20P family, and the Schuko RCD cords.
2. In-line, partway down the cord
The device is a module spliced into the cord, usually within a foot or two of the plug. The cord beyond the module and the load are protected. The short stub between plug and module is not, and it should be treated like any other cord: keep it out of the water. You reset at the module, which can sit where the user stands rather than where the outlet is. It suits equipment leads plugged into hard-to-reach outlets, hard-wired pigtails (open-end cords wired into a machine), and locking-plug cords where the plug head must stay slim to twist into an L5 or L6 receptacle.
On worldcordsets.com: the L6-15P in-line manual-reset and L6-20P families, and the 120 V 5-15P to open cords.
3. Rewireable (user-attachable)
The device has screw terminals and a cord grip instead of a moulded cord. You strip your own cable, land the conductors and close the housing. Whatever you wire downstream is protected, and you reset it wherever you installed it, as a plug head on your own cable or as an in-line module. It suits retrofitting an existing tool or extension lead without replacing its cord, odd lengths, unusual cordage (LSZH, high-temperature, oil-resistant), and export equipment where the plug pattern is decided late. The same cable takes a Schuko RCD or a UK RCD.
Match three things when you rewire. The cable diameter must fall in the cord-grip range printed on the device; too thin and the grip does not hold, too thick and the housing will not close. The conductor size must fit the terminals, and most accept 18 to 12 AWG (1.0 to 4.0 mm²). The rating must cover the load. A 15 A device on a 20 A load is the wrong device, not a tight fit.
On worldcordsets.com: user-attachable GFCI and RCD, plug-head and in-line, 120 V and 120/240 V, manual and automatic reset, plus the 230 V RCD versions for Schuko, UK and Australian plugs.
Side by side
| Plug-head | In-line | Rewireable | |
|---|---|---|---|
| Cord protected | all of it | all but the stub to the plug | what you wire downstream |
| RESET is at | the outlet | the module (you choose where) | where you install it |
| Fits a tight outlet | often not | yes (slim plug) | depends on the housing |
| Retrofit an existing cord | no | no | yes |
| Change the plug pattern | no | no | yes |
Ratings to match, whichever form you pick
Voltage: 120 V, 120/240 V or 230 V, and the device must match the supply exactly (see GFCI vs RCD). Current: the device rating must be at least the plug rating. A 15 A GFCI on a 20 A circuit is fine; a 15 A device feeding a 20 A load is not. Environment: outdoors and washdown want an IP-rated housing (IP44 splash, IP65 or IP66 washdown on RCDs; a "wet location" listing on GFCIs). Reset: decide it before you pick the form (manual or automatic).