Labs - Safety first

1. Labs - Safety first • Safety First! • Powerful rotating machinery and high voltage! • Accidents may occur, we must take precautions • Know basic first aid • Concentrate fully on the task, at all times • Watch your fellow student and warn him/her if necessary • You may be required to sign regulations for working in the lab Lecture 1

2. Labs – Safety first • Rotating machinery • Long hair – roll up hair under cap • No loose sleeves on shirt and jackets • Wearing tie or scarf in the labs is strictly forbidden • Touching moving parts is strictly forbidden • Never lean over running machinery • Stand back whenever possible • Do not touch anything unless necessary and stationary Lecture 1

3. Labs – Safety first • High voltage is dangerous • Make connections before turning the ”ON” switch • Turn swith ”OFF” before disconnecting equipment • As a general rule, always check voltage before you touch live equipment parts, even if you ”know” that power is turned OFF • Use insulated voltage, current and temperature probes • Never touch equipment parts under voltage (live parts) • Wearing rubber soled shoes is always an advantage • Never touch or lean onto metal machinery parts while making measurements with probes – then you may create a current path to earth • Wearing metal finger rings may be risky Lecture 1

4. Labs – Safety first • Low voltage is potentially harmful • In general you follow the same rules as for high voltage. • Wearing finger rings is risky • Example: A guy is disconnecting his 12 V car battery, using a steel wrench. He first disconnects the positive terminal, and his finger ring suddenly short circuits the wrench to the car chassis. The finger is burnt to the bone. • Examples of incidents with a 24 V DC /230 V AC power inverter • In the half bridge inverter the transistor switches are mounted on large cooling fins, not insulated from transistor collectors. You have moist hands and take a firm grip with the left hand on one and the right hand on the other cooling fin to compare temperature. Then you have 2 x 24 V between moist hands. The electric shock may be harmful – it is certainly painful, and you get dizzy and are breathing heavily: I still remember I did this in 1968: Have respect also for 24 V DC. • You touch a 230 V part of the inverter with one hand and the chassis/earth with the other hand Lecture 1

5. Labs – Safety first Working with inductors • Inductors give a voltage surge when current is suddenly interrupted • The voltage limiting mechanism is usually arcing through winding insulation breakdown • Typical surge voltage is 2 – 5 kV • Therefore: Swith circuit ”OFF” before disconnecting an inductor • If you are measuring ohmic resistance, short circuit the inductor before disconnecting instrument probes Lecture 1

6. Labs – Safety first Working with capacitors • Good capacitors keep their voltage for hours after being disconnected • Disconnect capacitors only after equipment is powered ”OFF” and capacitors have had time to discharge • Sometimes a meg-ohm discharge resistor is connected across capacitor terminals Lecture 1