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Author Topic: Lamp strikes, house lights go out Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-17-2010 09:09 PM                     one needs to llok at noise relative to ground on both the phase to ground and neutral to ground almost all scope probes are rated for 200v that I have come across The only time one has big issues if on wants to measure the rise time of a dimmer for mainly filtering which is a current issue as a rule so one needs to use a non inductive shunt and measure across it and then yes the chassis of the scope would be live If one is not confident with a piece of test gear or the safe procedures for working with it they should not attempt any troubleshooting of any electical device Again we know where the source of the problem is the lamphouse supression but finding the magnitude is the problem  |  IP: Logged Eric Robinson Jedi Master Film Handler Posts: 538 From: Santa Rosa, CA Registered: Jan 2005 posted 03-18-2010 12:22 AM                     quote: Tony Bandiera Jr It is EXTREMELY DANGEROUS if you aren't 100% sure of how to do it safely!! Your scope inputs MUST NOT have ANY reference to ground, and you MUST isolate yourself and the scope to avoid getting a severe shock. Yes your caution is wise. I think we have talked about this in the past, but could you refresh me on the danger by giving an example of an unsafe situation.  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-18-2010 01:20 PM                     Eric, one example would be in the case of needing to monitor a 208-240 volt DOMESTIC AC line (220-240 nominal overseas is a Line-to-Neutral voltage and doesn't apply to this example.), which has BOTH sides of the line HOT in reference to ground. If you connected the ground lead of your scope probe to one of those hot legs and your scope is either grounded through it's AC power connection (or if it's a battery powered scope and either you or it makes contact with a grounded object) you WILL cause an arc flash, burn things up or get a severe shock. Some older 120 volt AC equipment (TVs and radios were prime examples) also used to have one side of chassis connected directly to one side of the incoming AC power. If you happened to plug the cord in the wrong way you'd have the 120 volt HOT side connected directly to the chassis. Result was a hefty shock or for techs, destruction of test gear. Incidents like that are the prime reason the "polarized" AC plug was developed. But even that won't protect you against a miswired receptacle. In our cinema world, on the 120 volt side, there can be equipment where either old age, miswiring or a fault can put voltage on the chassis or points we would expect to be at or near ground potential. There are too many risks involved with 'scoping AC power, so that is why I always discourage it unless someone is throughly trained and has the proper equipment.  |  IP: Logged Eric Robinson Jedi Master Film Handler Posts: 538 From: Santa Rosa, CA Registered: Jan 2005 posted 03-18-2010 10:34 PM                     Hi Tony, Yes that sounds like a bad situation. Thanks for the heads up. So far I have been lucky. I guess you could call it safe if you measure the voltage with a voltmeter between ground(the test point you consider ground) and your scope leads ground to make sure you have zero volts before you commit to the measurement. This sounds safe doesn't it, or is there another catch?  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-18-2010 11:15 PM                     If your scope has a 3 wire power cord the shield of the probe is ground and will not be connected to the ac  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-18-2010 11:53 PM                     Eric, Yes taking a voltage measurement first wouold be a good safety precaution. quote: Gordon McLeod If your scope has a 3 wire power cord the shield of the probe is ground and will not be connected to the ac Gordon, why don't you take a wire from that ground and touch it to a hot AC line and tell us what happens. You are normally smarter that this, I hope you mistyped something or made that post without thinking first. With extremely rare exceptions, ALL AC power systems in buildings have a direct reference to ground (in the case of an average cinema, the supply transformer is mid-tapped to give the 120 volt to neutral voltage.) Bottom line is you gotta be super careful when trying to 'scope an AC power source.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 03-19-2010 07:36 AM                     If your scope has a 3 wire power cord the shield which is the ground lead of the probe is tied to the chassis ground of the BNC connector and the ground pin of the AC cord the centre probe is not tied to ground and has about 1Meg or greater impedance to ground  |  IP: Logged Tony Bandiera Jr Film God Posts: 3067 From: Moreland Idaho Registered: Apr 2004 posted 03-20-2010 02:06 PM                     quote: Gordon McLeod If your scope has a 3 wire power cord the shield which is the ground lead of the probe is tied to the chassis ground of the BNC connector and the ground pin of the AC cord.... And since the AC power source is referenced to ground you have the shielded side of the probe connected to the AC ground, and thus you have a connection to the AC side. If you need to look at the 240 volt source line-to-line as you would be doing here in the US, you will need to have the probe's shielded side connected to one of the hot legs...thus the danger and need for total isolation. It can ONLY be done IF there is TOTAL ISOLATION from Ground. Some folks use a 1:1 isolation transformer to do this, but the transformer itself can either hide or suppress noise (being essentially a choke coil) or introduce false noise (acting like and inductor) so sometimes direct connection is the only way to get a meaningful reading. quote: Gordon McLeod ......the centre probe is not tied to ground and has about 1Meg or greater impedance to ground That is 100% correct. Again bottom line is it is not something that should be done without proper equipment and training. I have both but do this only as a last resort when other methods of getting the problem solved fail. Back on topic,: Thomas, do as Louis suggested and let us know what happens. His suggestion makes sense. Another thing to check (I don't recall it being mentioned in this thread) is to make sure the lamphouse and rectifier are properly grounded and that there is a ground wire bonding the two together.  |  IP: Logged

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