SECURITY !
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For the repairing enthusiast (of old or recent sets), the security at the work is very important. Mains voltage is MORTAL. Voltages present inside tube appliances (radios, amps), but also in some semi-conductors assemblies (powerful amps, lights controls, switching power supplies, television sets,...) are dangerous too. |
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Always think that every appliance supplied by mains is dangerous when open.
The risk for the repairer of electical devices are: electrocution and short-circuit / overload.
Electrical shock:
Electrical shock can have two origins:
- Direct contact: The person touch an active part, normally under voltage of the equipment. Example: Main switch inside the set, main plug,... This is normally not possible on a appliance in order to work and closed. But, when repairing, it can happens!
- Indirect contact: The person touch an conductive frame, accidentally under voltage. For example: insulation leakage on a metallic frame, metal is anormally under dangerous voltage. Typical with old radios: defective capacitors between mains and frame (filtering caps)
Effects of electricity on the human body depends of some factors:
- Intensity: This depend of the body's resistance, and also of the voltage. Body resistance is not fixed, depending of the persons and also of some conditions like the state of the body (wet or dry), clothes (insulated or not)... In the baddest conditions, this resistance can fall under 500 Ohms. Under 230V, it gives a 460mA current! Generally speaking, if the current is more than 10mA it's difficult to lose one's hold.
- Contact time : The danger increase with the time of exposure.
- Current flowing : Heart region or not...
This is a relation between time of electric shock and current intensity. This table is valid for 50kg (or more) persons, when current cross over members.
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Zone1: usually no reaction Zone2: Not dangerous physical effect can appears (muscle contraction,...) Zone3: Usually no cardiac risk but from here, the risk of breath short exist Zone4: Cardiac risk exist Zone5: Real cardiac risk |
Cardiac risk means the vibration and beating of the heart's ventricles, stopping the blood flow... meaning the lack of oxygen in the blood, and quick death ...
Short-cut:
The effects of short cut can be dangerous for the service-man: explosion of components, fuses (see down in this page), melting brass projection....
When the short cut appears, the current into the wires and conductors is bigger than in normal use. Intensity of the shortcut current depends of some parameters: distribution transformer power, lenght and size of electric lines between the transformer and the wall plug.
In domestic use, this current has generally a value between 1000 and 3000 A. In the industry, shortcut currents can be 15000A and more.
The protection element (Fuse, magneto-thermic switch) take time (some milliseconds) to cut, conductors and appliances are crossed by this important current before the protection stop it, by opening the circuit. It's very important than those protective systems can cut without danger this violent current!
About fuses used on mains:
Fuses used in this application, directly connected to mains, must be able to cut the shortcut current. Glass «radio» fuses often used ARE NOT good for this use! This type of composant can cut without danger a maximal current of 10X nominal intensity. So, a 4A fuse can cut without risk a maximal current of 40A! It's obviously too low, refer to the current present at the short cut moment. What's the risk? Fuse's explosion, with little part of glass throwing up in all directions... If the fuse is installed in closed style holder, not really risk. You shall not use other fuses than ceramic HBC types (High Breaking Capacity), able to cut 1500A (or more) current. This kind of fuses (left on the photograph) are more expensive than glass models (right on the photograph), but don't explode.
Short cut effect in a switching power supply:
(clic to enlarge..) The original glass fuse and the PTC exploded. All little glass parts were present everywhere in the box.
Overload:
Overload of wires or appliance (transformer,...) will produce over heating. If the electical protection is false, the circuit will heat a lot, and can burn ! Injury risk by contact with overloaded elements is real. For information, in Belgium, many domestical burn down of houses (produced by electricity) results of lighting transformers overloaded and badly protected...
Examples of what you can find in old appliances!
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In this 1936 amplifier, transformer fuse is replaced by a wire...In case of overload or shortcut at one secondary, this transformer will burn! |
Mains fuse from an old radio...As you see, it won't be useful if overload or shortcut! |
Protection of persons and appliances under test:
Mains wall plugs of a repairing place, where appliances are open, should be protected by a high-sensibility leakage switch. 500 or 300mA protections are not useful for the human body protection. Wall plugs where objects under repairing are connected must be supplied by means of 30mA or less leakage switch. I've choose a 10mA switch, for my lab. It's low, yes. But every appliance, very old or personnal assembly, MUST pass the test and not made the switch cut: think at the security of the future users!
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Laboratory wall plugs protection: magnéto-thermal protection and leakage switch in the same unit.Thermal value 10A , B curve (sensible magnétic) and 10mA leakage switch. This unit should be checked one time per month, normally! |
My security system
To work insulated from mains voltage, I've built this unit: (clic on to enlarge..)
One view of the realisation on the laboratory page...
One 1000VA insulation transformer is mounted at the entry. The «variac» is connected at the secundary of this first transformer: this variable transformer does not provide insumation by himself! Volt and amp meters; outputs on 16A plugs. The protection is a 4A magneto-thermal unit. S2 allows to shortcut the 1A amp-meter and S3 switches the volt-meter on fixed or variable outputs.
Advantages:
Insulation: When connected at output of this system, the appliance under test is totally insulated from mains. A contact with one of the wires did not creates an electrical shock. Voltage is always dangerous between the two wires! Other advantage is the possibility to connect without danger an oscilloscope to a circuit directly connected to mains, for example: light controls, switching power supply, all-current radio,...
Short cut current limitation: The insulation transformer limit the current, in case of shortcut in the connected unit. Transformer used in my assembly «gives» 174A when short-cut. It's less than the mains...
Voltage adjust: The second transformer allows a progressive and continous adjust of the output voltage. It's well for the tests and to supply very old appliances, unused for very long time. In this case, you should check the current.
The disadvantage of this insulation is, in case of problem, the leakage switch installed before will not operate!
And,...
This page can seems to be grim and morbid! However, it's only basic information, generally given at persons who work in the industry. Scaring is not the idea of this page, but to catch the eye at the dangers in this hobby! We can also think at:
- Keep a place only for repairing works. Living-room or kitchen table aren't good places! In any cases, our ladies did not give agreement!
- Completely check unknown appliance before plug it for tests!
- Never leave on without watching a device if you're not sure of its reliability.
- Keep children away of the repairing area, when works are done (and open assemblies supplied).
- Install minimal protection equipment...
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