Hi folks. In sc here. Public water. No expansion tank on my water heater. When I turn on a faucet the water pressure drop after about a second. No other fixture is open. Happens all the time. How do I retain the water pressure at my fixtures from the time I open them until I close them? Happens in all fixtures.

when you say "all fixtures" do you mean toilets, showers and outside faucets?

usually, it is caused by a defective pressure reducing valve in the system

Yes. Everything.

I will have to Look for one tomorrow. No regulator in the house. If there
Is one out in the street key pit - who is responsible to
Replace it?

Has the main shut-off valve been checked, to see if it's fully open? What type of valve is the main water shut-off? If a gate valve, maybe it's broken.

Would installing a pressure expansion tank help keep this water pressure consistent? I do not
Have one over my water heater.

It might help you sometimes and seeing less of an initial drop for at least 1 of 2 reasons:

Most likely you have a check valve in your line and so what might be happening is that most public pressures rise and fall and are not perfectly static at one pressure level and so when you're not running your water and not being used there might be more pressure in your static line at the time that you turn on your water which is coming from your public source. An example might be that your public source might fluctuate anywhere from 50-75 pounds. IF you have a check valve inline and aren't running the water .... at some time your line will be holding the highest 75 pounds of pressure and when you turn on your water that 75 psi will come out of your faucet initially and then go to whatever the current source's pressure might be such as 60 pounds.

Also as your water is heating if you have a check line in your line someplace feeding the water heater (such as at the meter or a pressure regulator) your pressure will rise in your house water line and that is the main reason for an expansion tank to take on (absorb) the extra expansion.

So as a follow up. I installed a 2 gallon expansion tank on the cold side of my water heater. I left it at factory pressure of 25 psi. The problem has been eliminated. I now turn on a fixture and the pressure seems to stay consistent. Thanks all for you wisdom!