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Author Topic: What's the best way to determine the quality and stability of your AC power? Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 01-16-2005 03:08 AM                        My scope is allegedly rated for up to 400VAC or DC input. Especially with an X10 probe which reduces the signal to 1/10, it should be safe. But before I try that, I will dig up the manual and see if there is a schematic of the VA though.  |  IP: Logged John Anastasio Master Film Handler Posts: 325 From: Trenton, NJ, USA Registered: Sep 2000 posted 01-16-2005 07:18 AM                        Yeah, so's mine, but if you plug it directly into the AC mains, you'll blow the breaker because of the way it's internally connected. Be careful. We once had an enormous problem with our 50Kw FM transmitter at a station I was employed by. The darned thing went off the air every day at the same time. We had the power company put a line monitor on it and discovered that someone was using an old freight elevator in the building every day at that time and the line surge from the motor kicking in was enough to trip our breakers.  |  IP: Logged Oscar Neundorfer Master Film Handler Posts: 275 From: Senoia, GA Registered: May 2000 posted 01-16-2005 07:20 AM                        I am going to STRONGLY recommend that if you don't know EXACTLY what you are doing, DON'T connect your scope to the AC line. At best, you may fry your scope, and at worst you may fry YOU. Most scopes I have seen have the ground side of the probe connected to scope chassis ground which in turn is connected to AC ground. Accidentally connecting this to AC hot will result in lots of smoke and flames and a huge repair bill. And DON'T lift the AC ground of your scope either. You could have your scope chassis floating at 120 VAC which could hurt you bad if you got between it and ground. Even though you may see some noises on a scope looking at the VOLTAGE waveform, it is not all that informative anyway. If, on the other hand, you can look at the current waveform through some type of AC current probe, it is amazing to see the huge spikes and distortions that occur. The source of these problems is numerous, with switching-type power supplies and industrial motor drive controllers being big culprits. In some large buildings, three-phase transformers have burned up due to excessive currents in the neutral leg caused by too many harmonics which upset phase relationships and increased the neutral current way beyond specification. Power companies have reaaly been looking hard at the amount of distortion and noise acceptable on power lines and are establishing standards for this. A lot of more modern power supplies are striving for a power factor approaching 1 to limit the reactive loads presented to the distribution system. Noise, transient events, and distortion are not good for electronic equipment. But transient events such as large spikes are probably the most damaging to the equipment in your booth. With todays microprocessor-based equipment in the booth (processors, automations, etc.) it is always good practice to install some type of line conditioner to prevent errors in the digital circuits which can result in strange and unexplainable behavior such as a processor shutting down or losing its settings or an automation doing something strange. Unfortunately, this is an extra cost item, and this is protection that many are not willing to purchase. Surge protection such as Mark G. describes is a very good idea. Also, contact your power company too see what whole building protection might be available. It seems to me there are some non-degrading types of surge protectors available for whole building protection. I believe they are called gas-discharge types, but not sure.  |  IP: Logged Michael Schaffer "Where is the Boardwalk Hotel?" Posts: 4143 From: Boston, MA Registered: Apr 2002 posted 01-16-2005 12:46 PM                        Thanks for the tips. Figuring out which line is hot and which is neutral, apart from that, are there any other hidden dangers? Am I mistaken in assuming that an X10 probe should greatly minimize the risk? Using an AC current probe is a very good idea. I have a Fluke clamp meter which outputs the current readings as a voltage (1A=1mV). You plug it into your multimeter and read the current as a voltage. This should be safe to hook up to the scope. Although I don't know how accurately it represents the actual wave form of the current, or if it "rounds off" the input to give a stabilized reading. I have no desire to blow myself up. Last night I was working on a power supply and I discovered that some idiot had bypassed the breakers on the front of the console. They were not even bad, I have no idea why. I made sure that all the breakers in the distribution panel were off and tested all the lines before starting to work on the power supply. Later when I grabbed the AC lines to hook them back up to the PS, there was a puff! and a flash! which resulted in a nice burn mark on my hand. It turned out that the projectionist, who knew I was working on the console had turned the breakers back on in the meantime. It turned out that he "thought they belonged to another screen". Why he thought so even though they are clearly labeled, and why he felt the need to turn the breakers on even though all other screens were running - would they run if their breakers were off? -, he couldn't explain to me.  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-16-2005 02:02 PM                     quote: Michael Schaffer I have a Fluke clamp meter which outputs the current readings as a voltage (1A=1mV). You plug it into your multimeter and read the current as a voltage. This should be safe to hook up to the scope. Yes, but may not be the same as looking at the AC directly although it will be interesting to loko at. Also, the use of a storage scope can be very helpful as it can be set up to trigger and record a large spike so it can be easily seen. As Oscar mentioned Motor drives and switchers can put alot of crud back on the AC and this definately can be seen with a scope as RF riding along on the hot lead. If in doubt buy a small isolation transformer as this will make looking at AC perfectly safe even if your scopes input is grounded. In this case the primary AC is not directly connected to either chassis or the actual input. It is the secondary of the transformer you're looking at. Mark  |  IP: Logged Phil Hill I love my cootie bug Posts: 7595 From: Hollywood, CA USA Registered: Mar 2000 posted 01-16-2005 02:49 PM                  quote: Michael Schaffer It turned out that he "thought they belonged to another screen". Why he thought so even though they are clearly labeled... That's why you should ALWAYS use lock-out tags or at least put tape across the breakers and a note giving your name, location, and saying not to energize...people are working... etc. Also, It is extremely dangerous to connect a scope directly to the AC line. Even if you make sure the neutral is on the "grounded" side of the scope input, there is a substantial risk of blowing it up anyway. There is a significant possibility of a low voltage/high current difference between the neutral and ground...especially in large distribution systems as in malls and complexes. I once saw a guy lose a hand when his wrench disintegrated when it accidentally hit the ground bus while he was tightening a neutral connection on a disconnected "dead" 1200 Amp standing section. DANGER! DANGER, WILL ROBINSON! Again, I STRONGLY warn against it, but IF you *MUST* connect a scope in that way, make sure you use a ground lifting adaptor on the scope's AC plug and put the scope on an insulated surface and DO NOT touch it...or leave it unattended. Make sure the neutral side is connected to the "ground" side of the scope. And, just because your scope is "rated" for 400vac, that really doesn't mean much as many times there are kV spikes on the line that will fry your scope. There are plenty of commercial recorders, loggers, and chart recorders available to buy or rent made specifically for monitoring the AC line. Get one!  |  IP: Logged Mark Gulbrandsen Resident Trollmaster Posts: 16657 From: Music City Registered: Jun 99 posted 01-16-2005 04:42 PM                     In actuality looking at the 120 volt AC power line should be no more dangerous that looking at the output of some very high powered amplifiers who's output can easily exceed what a 20 amp AC line can carry. Here is a shot of the AC in my house off the screen of my TEK 465B. Pretty clean except for some distortion at the positive and negative going peaks. This distortion is typical in this area but varies somewhat from location to location. This was done with the scopes AC plug ground pin connected as normal. I never advocate the floating of any ground in any piece of AC operated equipment because its very dangerous to do. If you're unsure about your scope then get a 465B for cheap off of E-Bay. It works just fine for this purpose. Mark  |  IP: Logged

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