I'm currently in Peru and have a three-pronged travel adapter. This was fine in Colombia but the plug sockets here mostly have inputs for two prongs.
I want to plug my UK Macbook charger (which fits Peru voltages) into my three-pronged adapter and then that into a two-pronged adapter. Is this safe? I can't find a UK-to-two pronged adapter where I am in Peru.
Yes, it's generally safe. Adapters don't have any active components, so assuming they're all rated for the voltage/amps you're putting through, there's little risk of overload etc like there is with transformers.
The main catch is that if you plug a three-pin plug into a series of adapters that "loses" the third pin, your device will no longer be grounded. (Then again, this can happen with a single "wrong" adapter as well.) Also, while more annoying than dangerous, stacks of adapters tend to be fiddly and come loose easily.
In the specific case of a MacBook, it's safe: the MacBook power supply is non-grounded. The grounding pin on your UK "type G" plug is probably a plastic dummy pin, so going from a UK "type G" to a Colombian three-pin "type B" to a Peruvian two-pin "type A" or "type C" doesn't lose the ground connection.
If, on the other hand, you're using a device that requires grounding, it's only safe if the "Peruvian two-pin" plug is a "type F" (Schuko) plug. "Type A" and "type C" plugs don't have ground connections, and you risk getting electrocuted if, for example, a wire inside the device breaks and comes in contact with a metal case.
Stacking two of them isn't really the issue. It's that an awful lot of that stock is made in the far east, and lacks any certification by any competent testing lab. (in fact the CE mark can be counted on to be counterfeit unless an EU bricks-and-mortar manufacturer made or imported it).
So look for ones with the stamp of a reputable testing lab. These shops don't bury the lede: If they have a hard-won UL Listing, ETL, BSI, CSA, TUV etc. -- they'll put it front and center. However if the most prominent mark is CE or CCC, that means they couldn't get a proper listing; it's a self-admission of junkness.
It also depends on the load, in two axes.
If you see a "Square on Square" symbol (literally a square inside a square), that means your load is double-insulated. That will help compensate for adapter problems; not least that these adapters love to exchange hot and neutral. [not least, British BS1363 (Type G) puts hot on the left (ground down), US NEMA 5 (Type B) puts it on the right.]
Adapters are notorious for very flimsy connections. You can get away with murder if your load is only a 10 watt iPad charger. But trying a 1500 watt hair dryer is asking for trouble. The issue is "series arcing faults", where the power jumps (arcs) across a flimsy connection in order to get through your device. The more current, the more arcing; shut off the load and arcing stops. An arc-fault detecting circuit breaker will help protect you, but those are rare.
A GFCI aka RCD/RCBO device will help keep you from getting shocked by a loose or broken adapter. It won't do anything to prevent arc-caused fires, the aforementioned arc-fault breaker (AFCI) would do that.
Either protection could be located anywhere in the building; the proof of the pudding is if you hit its TEST button, the outlet loses power. It's generally at the receptacle itself, or at a nearby receptacle, or at the circuit breaker; all are equally acceptable.
Earthing (grounding) is covered in the other answers but the other thing you should make sure of is that there isn't too much mechanical stress on the adapters or on the socket that you're using, as that could lead to a poor contact which could overheat.
If the socket is trailing (e.g. on the end of an extension lead) you can lie it down so the MacBook adapter is on the floor. If it's a wall socket I would probably try and support the MacBook adapter from underneath with some object so as to take the weight off the socket. Remember the MacBook adapter gets warm, so don't cover it up.
I'd also suggest not leaving this arrangement unattended while plugged in.
Travel adapters are problematic in several ways.
Stacking adapters, and/or stacking wall-warts on top of adapters can compound the issues, there are more (often-questionable) contacts to fail, more chance of losing the ground, and more mechanical stress on the flimsy, poorly fitting connections from the extra weight/leverage.
Having said that I would take a stack of two good-quality well-fitting adapters over a single low quality poorly fitting one any day. Indeed Swiss brand Skross actually sells some of their adapters in the form of a stack, to accomodate the "Schuko" side-contact earthing system.
Safe with care.
Main issues are
What others said - lack of ground MAY matter.
Check on a case by case basis.
Unstable stack may expose contacts with shock risk.
Easy to do accidentally.
This will not be "too" common but definitely should be something you are aware of.
Substantial currents through poor contacts could create a fire risk.
Unlikely with laptop level loads.
Such daisy chaining will work fine in practice for reasonable loads, but it's not recommended, because you worsen the short circuit protection. Be aware that the short circuit protection equipment is designed to trip at a rather high current (e.g. a typical 16 A fuse is allowed not to trip until you reach 30 A).
If you plug your notebook charger in a chain of adapters and that charger fails with an internal short circuit, the total resistance of the contacts of several adapters may be enough to keep the current limited. If the current stays below 30 A, the protection may never trip, and the total dissipated power of 230V*30A = 6.9kW will quickly set your adapters and the charger on fire.
Note that it doesn't matter how low the nominal load is in this case, the only thing that matters is the maximum power the mains outlet is able to deliver. Indeed, if the load is high enough, a stack of daisy chained adapters can overheat and catch fire at a nominal current without any fault in the load, but that's a separate issue which doesn't affect laptop chargers.
Also, most laptop chargers (MacBook included) are double-insulated and don't need protective earthing to be safe. But if you have non-insulated equipment you plan to plug into the adapter, like a toaster or a hair drier, don't use any adapters which would break the protective earthing connection. Any adapter with only two prongs is guaranteed to break it, with 3-pronged adapters you have to verify that all three contacts actually match. The protective earthing will allow the protection to trip the moment your equipment starts leaking current into the earth terminal, not the moment you touch it and it starts leaking current into you.
So, be careful, only use adapters which make good contact (if you hear noise while powered it's a bad sign) and don't leave the contraption unattended. Or better yet, buy a single adapter.
