Why is it not safe to connect ground to neutral in a receptacle?
The third pin, the "ground pin," is connected to the external metal parts of whatever you plug into the receptacle. The other two pins (or "prongs"), the "hot" and the "neutral" as they are sometimes called, are only connected to stuff that is inside the case. But if one of them were to break and come into contact with the case, it might give a shock to whoever next touches the case.
The purpose of the ground pin is to take that shock current from the case and send it directly back to the source. This current will be high enough to cause the circuit breaker to trip. In other words, the ground pin will save the life of the person touching the energized case.
If you have only a 2-prong receptacle (no ground pin), the protection you lose is the ability of the system to trip the circuit breaker. So whatever shock a person might get from the case will not be stopped, unless they are lucky enough to be able to pull their hand away, or perhaps they might fall on the floor (thus causing their hand to be pulled away from the case).
So what happens when you connect the ground and the neutral within a receptacle? You cause the case of whatever is plugged into that receptacle to become energized. How? By virtue of the fact that there is current flowing in the neutral wire, the fact that the neutral wire is connected to the ground wire, and the fact that the ground wire is connected to the case of whatever item you have plugged in.
Every time anyone touches that case, they will receive a shock. But that shock is usually very slight ? so slight that the person might not notice. Therefore, the person might continue to operate that equipment for years, and still not know they are in danger. In other instances, the person will not be so lucky. The amount of the shock might be low one day and higher the next day. It cannot always be predicted.
An extreme danger comes into play, when a neutral wire becomes loose or is disconnected. In that case, the only path current can take from the equipment back to the source is along the ground wire. That means that the person touching the case will get a much greater shock, and this type of accident has been known to be fatal.
NOW HEAR THIS: If you plug something into an outlet that has the neutral and ground tied together, and if that something has metal parts on the outside, then as soon as you touch the thing there is going to be some current flowing through your heart. Might not be much, but it will happen. Every time. No exceptions.
Would you be happy about that?
