48 V AC control

48 volts is pretty safe. It takes at least 30-some volts under ideal conditions to prove lethal, ideal conditions being broken skin so as to reduce resistance to current flow. The frequency does play a role. Low frequency AC can be harder to let go of if you get shocked. Really high frequency AC will go right around you; it's called the "skin" effect because it won't actually penetrate your body (so much less dangerous than low frequency AC). Tesla hooked himself up to high frequency AC at one of the World's Fairs and thrilled the audience with a corona around his body and his hair standing on end. DC can bite you good, but you'll let go right away. In the good old days, control voltage was generally 24 volts, which I would consider safe.

48 volt is not safe especially if the source of 48 volt can supply high power current.(for example 4 car battery).

Yeah, sure. Why don't you see how much current it takes to kill a person? The quantity is somewhere between tiny and miniscule.

Please be specific about "Safe". Where the controls are located in panel or hand held control switch. Will it be accidental contact? or regular hand contact?.

Any voltage over 30 V is considered potentially dangerous.

The International electro technical commission provides curves for the tolerable body current as a function of frequency and for capacitive discharge currents (IEC 60479-2, 1987-03).

Effects of an electric current passing through the vital parts of a human body depend on the duration, magnitude and frequency of this current. The most dangerous consequence of such an exposure is a heart condition known as ventricular fibrillation, resulting in immediate arrest of blood circulation. Humans are very vulnerable to the effects of electric current at frequencies of 50 or 60 Hz. Currents of approximately 0.1 A can be lethal.

Effect of 50/60 Hz. current range on human for duration of 1 sec.:

10 to 20 mA Experience pain, sustained muscle contraction

30 - 75 mA Painful and severe shock, muscular contraction, difficulty in breathing

100 - 300 mA Experience heart fibrillation

Considering the above consequences, 24 V AC may be considered as safe voltage particularly for portable instruments or hand lamps in confined locations. For control circuit applications however there are other limitations which may force the users to adopt even 110V AC as control voltage.

The 48 value is probably the RMS value and as such the peak to peak value is

2*48 V*1.4142 = 135.7 V which is quite high. If there is no current limiter I am sure it could be a shocking experience