Re: Lightning Protection for PA System
Scott, Not knowing how your system is "exactly" built makes it difficult to second guess the lightning. However, here is one way to protect your amplifiers: Move the protection interface out to the edge of your building. Use rod located there with all referenced to ground. At this same location use a 10A AC mains line filter between the amplifier and the speakers (cheap filtering which should be able to pass the audio) with AC line towards amp and load towards speakers. Between filter and speakers place fusing in series (won't do much for truly high voltage which will jump, but will take care of a lot of nuisance discharges) place gas discharge tubes there. [ If you can get a surplus telephone entry block, the type with the carbon "shorts", they work great here, too. ] Back side of filter place tranzorbs, then back at amp place more tranzorbs. You prevent lightning damage by designing a filtering system which limits the maximum amplitude that can get into your electronics. And the most effective rejection filter is always high impedance in series, low impedance to ground, high impedance in series and low impedance to ground, ad nauseum. Just make multiple paths that do this and you can even sustain a direct hit. Speakers and wires will probably fry though. - Robert - Robert A. Macy, PEm...@california.com 408 286 3985 fx 408 297 9121 AJM International Electronics Consultants 619 North First St, San Jose, CA 95112 -Original Message- From: Scott Lacey To: emc-p...@majordomo.ieee.org List-Post: emc-pstc@listserv.ieee.org Date: Saturday, June 15, 2002 9:19 AM Subject: Lightning Protection for PA System To the group, I am seeking advice as to the best methods of protecting a Public Address system against recurring lightning damage. The system uses several commercial PA amplifiers, each driving several speakers at indoor and outdoor locations. There are also several locations where microphones can be plugged in. The longest speaker wires may be up to 250 yards long. It is believed that the charge is being coupled to the speaker wires where it then returns to ground at the amplifier location within the building, destroying the solid state devices within the amplifier. A technician has added fuses to all external microphone inputs and speaker outputs. While these have blown several times during storms without obvious damage to the amplifiers it is my belief that fuses are generally too slow to protect semiconductor devices. I am seeking advice as to surge suppression devices. System particulars are as follows: 1) The PA amplifiers have 70 volt outputs. All speakers are transformer coupled. 2) All microphones use standard XLR connectors. They plug into metallic conduit mounted jacks at locations inside and outside the building. The outside microphones are unplugged during storms. 3) The amplifiers are located on the second floor of the building. Each amplifier is dedicated to a set of speakers at one location. The amplifier driving the longest wires is the one which most often has to be replaced. 4) The building is in a location known to be susceptible to lightning activity. Electrical appliances have been destroyed on at least two occasions. 5) All protective grounding efforts to date have been made to the conduit. I am recommending that this be supplemented by driven rods. It is pretty easy to buy commercial surge suppression devices for the ac lines. I feel I need advice as to the best methods to protect the audio inputs and outputs. For the 70 volt outputs I am thinking of using gas tubes to earth where the wires enter the building supplemented by MOV, Tranzorb, or other devices near the amplifier location. I welcome suggestions as to device types. For the microphone inputs I am thinking of using semiconductor transient voltage suppressors near the amplifier. Again, any suggestions are welcome. Thanks in advance for any advice and guidance. Scott Lacey sco...@world.std.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
Re: Lightning Protection for PA System
Scott it sounds like you have a tiger by the tail in that you are in a particularly hot lightning location. Your efforts to date sound reasonable. I will expand on a few statements. First, your system is a good deal like a telephone system and many of the techniques used in telephony may help you. You might like to check out the US Printing office for REA documents relating to lightning protection. For the 70 volt outputs I am thinking of using gas tubes to earth where the wires enter the building supplemented by MOV, Tranzorb, or other devices near the amplifier location. I welcome suggestions as to device types. Tranzorbs are a very good product. They also come in a wide range of voltages which make them applicable to almost all applications. They are fastest devices available and the only ones that stand a chance of reacting to the leading edge of the current pulse. MOVs are the heavy lifters but are generally too slow to offer much leading edge lightning protection. They are labeled transient protectors as opposed to lightning protectors for this reason. Gas protectors fall in the middle for speed and are only available for higher voltages. All of these devices will fail in time if subjected to repeated lightning strikes. Diodes (Tranzorbs) fail shorted. Gas tubes fail open. MOVs may fail open or low resistance. Ordinary silicon signal diodes can be used to protect microphone inputs. Fast shockly power diodes can be used too. Remember the bigger the diode the slower it reacts. All of the above components are network devices meaning they are not designed to absorb the total energy of the lightning strike by themselves. They are designed to work in conjunction with other components in the protection network. The protection components, particularly diodes, need to react with some resistance or inductance to limit current. The copper resistance of the wire and transformers may be enough but adding a few ohms more, say 4.7 ohms, 1 or 2 watts, in series with each side of the line, will usually allow the diodes to survive a direct hit. Inductors and inductive resistors (wire wound) sound like a neat idea since inductance will slow down the wave front and provide an impedance for the protection devices to react with. In reality inductors do not survive well. This is because magnetostrictive reaction, particularly in solenoid wound resistor or inductors, can explosively shatter the coil form. However, I have seen dual segment wound, toridal, common mode chokes successfully used. The ferrite cores do not shatter. Good luck, Fred Townsend Scott Lacey wrote: > To the group, > > I am seeking advice as to the best methods of protecting a Public Address > system > against recurring lightning damage. The system uses several commercial PA > amplifiers, each driving several speakers at indoor and outdoor locations. > There are > also several locations where microphones can be plugged in. The longest > speaker > wires may be up to 250 yards long. It is believed that the charge is being > coupled to > the speaker wires where it then returns to ground at the amplifier location > within the > building, destroying the solid state devices within the amplifier. A > technician has > added fuses to all external microphone inputs and speaker outputs. While these > have blown several times during storms without obvious damage to the > amplifiers it > is my belief that fuses are generally too slow to protect semiconductor > devices. I am > seeking advice as to surge suppression devices. > System particulars are as follows: > > 1) The PA amplifiers have 70 volt outputs. All speakers are transformer > coupled. > > 2) All microphones use standard XLR connectors. They plug into metallic > conduit > mounted jacks at locations inside and outside the building. The outside > microphones > are unplugged during storms. > > 3) The amplifiers are located on the second floor of the building. Each > amplifier is > dedicated to a set of speakers at one location. The amplifier driving the > longest wires > is the one which most often has to be replaced. > > 4) The building is in a location known to be susceptible to lightning > activity. Electrical > appliances have been destroyed on at least two occasions. > > 5) All protective grounding efforts to date have been made to the conduit. I > am > recommending that this be supplemented by driven rods. > > It is pretty easy to buy commercial surge suppression devices for the ac > lines. I feel I > need advice as to the best methods to protect the audio inputs and outputs. > > For the 70 volt outputs I am thinking of using gas tubes to earth where the > wires > enter the building supplemented by MOV, Tranzorb, or other devices near the > amplifier location. I welcome suggestions as to device types. > > For the microphone inputs I am thinking of using semiconductor transient > voltage > suppressors near the amplifier. Again, any sugges
Lightning Protection for PA System
To the group, I am seeking advice as to the best methods of protecting a Public Address system against recurring lightning damage. The system uses several commercial PA amplifiers, each driving several speakers at indoor and outdoor locations. There are also several locations where microphones can be plugged in. The longest speaker wires may be up to 250 yards long. It is believed that the charge is being coupled to the speaker wires where it then returns to ground at the amplifier location within the building, destroying the solid state devices within the amplifier. A technician has added fuses to all external microphone inputs and speaker outputs. While these have blown several times during storms without obvious damage to the amplifiers it is my belief that fuses are generally too slow to protect semiconductor devices. I am seeking advice as to surge suppression devices. System particulars are as follows: 1) The PA amplifiers have 70 volt outputs. All speakers are transformer coupled. 2) All microphones use standard XLR connectors. They plug into metallic conduit mounted jacks at locations inside and outside the building. The outside microphones are unplugged during storms. 3) The amplifiers are located on the second floor of the building. Each amplifier is dedicated to a set of speakers at one location. The amplifier driving the longest wires is the one which most often has to be replaced. 4) The building is in a location known to be susceptible to lightning activity. Electrical appliances have been destroyed on at least two occasions. 5) All protective grounding efforts to date have been made to the conduit. I am recommending that this be supplemented by driven rods. It is pretty easy to buy commercial surge suppression devices for the ac lines. I feel I need advice as to the best methods to protect the audio inputs and outputs. For the 70 volt outputs I am thinking of using gas tubes to earth where the wires enter the building supplemented by MOV, Tranzorb, or other devices near the amplifier location. I welcome suggestions as to device types. For the microphone inputs I am thinking of using semiconductor transient voltage suppressors near the amplifier. Again, any suggestions are welcome. Thanks in advance for any advice and guidance. Scott Lacey sco...@world.std.com --- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
