How can they do that?

- G
- Gerald Newton
  
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posted
19 years ago

Fri, Apr 2, 2004 6:27 AM

I bought a portable lamp from Fred Meyers that is UL listed. It has parallel metal rods extending up to the lamp from a base that contains a 120 volt to 12 volt transformer that is not a listed Class 2 power supply. I accidentally mashed the rods together and the lamp went out. I found an open five ampere fuse in the base. I thought the wires went up inside the small tubular bars but lo and behold, no! The bright metal parallel rods are the conductors. I replaced the fuse and now this lamp sits on my desk with exposed bright metal rods with 12 volts between them.. Somehow I find this weird. The lamp is made in China.

- J
- <jwzumwalt
  
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posted
19 years ago

Fri, Apr 2, 2004 7:29 AM

Anything lower than 50volts is considered "safe" by the electronic industry and does not necessarily require special safety features. Also, A/C is considered more dangerous than DC.

You will note that many cars have exposed 12V terminals on the battery and you can grab both terminals without being "bitten" :-)

Another good example is the telephone circuits in a home may have up to 35V A/C but are considered "safe" and do not require safety protection.

JZ

- N
- ns
  
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posted
19 years ago

Fri, Apr 2, 2004 2:12 PM

This is wrong: SELV - as defined by the safety agencies - is defined as 42 V AC rms or 60 V DC (~42 * sqrt 2). When you shorted the conductors, you were saved by the fuse, put there intentionally to protect the user and equipment from over-heating.

When a reputable car manufacturer designs a car, they will take into consideration the chances of someone actually bridging the battery posts. A good design will include post covers to prevent accidental short (that can be a serious thing).

On the telephone side (PSTN): the voltage between Tip & Ring when in on-hook position is -48 V DC nominal (this is dependent on your distance from the CO or RT, and what wire gauge was laid). It is negative with reference to ground. This voltage has a current limitation of 30-60 mA. On ringing, you have ~90 V AC (20 Hz) super-imposed on the -48 V DC, so there is a chance you will see up to 130 V momentarily.