Bring 110 volt to 90 volt

Here is the problem with using a light bulb in series to limit current.

Remember, it's the CURRENT that is the hazard, not the voltage.

GFCI limits current to around 5 ma for personal safety.

50-100 ma can be lethal.

Although the human body can have quite a difference in resistance, a fair estimate is around 1500 ohms.

Let's say the bulb cold (another problem, the resistance of the bulb changes when it heats up) is 100 ohms.

So, a person grabbing the 120 volt circuit with no bulb will cause 80 ma to flow. If the bulb is added in series, the amount will only drop to 75 ma. Both still in the danger zone.

Also, if you do the math, a 100 ohm series resistor will only drop the voltage across a 1500 ohm person to 112.5 volts (from 120).
 

kwired

Electron manager
Location
NE Nebraska
GFCI limits current to around 5 ma for personal safety.
And 5mA is enough to get your attention in almost all cases. Anyone that doesn't believe that - put yourself in series with one of those GFCI testers and push the test button.:happyyes:
 
25V buck boost transformer

doug - In all due respect, there is not one of us that has a clue what you are trying to do.

ice
I think I understand.

Just like anywhere else, the lab cubicles have 120 volt receptacles and as such, have 120 volts.

The OP is concerned that the testing of projects would expose the high school students to a potentially lethal voltage.

First of all, there should be no high school projects that involve 120 VAC. The receptacle is there for test equipment, computers, and for power supplies. That being considered, putting a light bulb in series with the receptacle may render such equipment erroneous, non-functioning or subject to damage from extended low voltage use.

Exposing teachers to 120 VAC without proper PPE is an OSHA violation. I don't know if that extends to students, but teachers are employees.

Any high school project that will work on 120 VAC can easily re-designed to work on 12 or 24 VAC so there is no reason to use 120 VAC.

If any of the projects involve things like Tesla coils, the 120 VAC feeding the transformer is the least of your safety concerns.

The OP mentioned that the receptacles are GFCI. That is the best protection that one can use. The series resistance idea won't limit current to a safe range and will pose problems for connected equipment and devices.
 

kwired

Electron manager
Location
NE Nebraska
The OP mentioned that the receptacles are GFCI. That is the best protection that one can use. The series resistance idea won't limit current to a safe range and will pose problems for connected equipment and devices.
Limiting voltage to the GFCI logic circuitry could compromise the functioning of the GFCI also.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It's 110vac thanks . And what's with the negative comments from the others.
I'm trying to keep contact voltage down when testing there projects . Try thinking from A to Z.
I thought this site had class. Those of you that are bothered by question don't reply.
I ask for help , guess not everyone knows everything you gentleman know. So one more time. Students , voltage , safely.
The difference between getting killed by 110V and getting killed by 90V is lost on the victim's family...

The reason for the earlier derision is that you did not post your reason up front, and for the life of me, I too could not think of a valid reason to want to drop 110V down to 90V, when anything that is designed to work on that voltage will not then work right. Standard utilization voltage at that level is 115V, +-10% is common to allow for places that only have 110V for instance. So at 90V, you would be well outside of that -10% range of acceptability. Now that you post your reasoning, to be quite honest it is still not valid. There is no REAL difference in the safety of 90VAC compared to 110VAC if a student comes in contact with it. Both levels are potentially lethal.

The only point at which it gets significantly LESS dangerous is what is referred to as "limited energy" which has is based on voltage AND current available in the circuit. 30V is the commonly accepted maximum voltage for that, but 12V at 100A is still very dangerous. So there are different "classes" of limited energy defined in the NEC, article 725 (I think). If the lab requires the potential risk of contact by the students, I would keep it within those limits AND back up the entire situation with a GFCI feeding the transformer supplying that limited energy.
 

petersonra

Senior Member
As others have said:

There is little difference in the lethality of 90V versus 120V.

Allowing high school students to work around 120V (or 90V) while it is live is just asking to kill someone, especially if they have been incorrectly promised that it is "safe". I might add it is also a bad idea for electricians as well unless they can qualify for an energized work permit and are using the proper PPE.

Best bet for basic electrical experiments is to use a lower voltage - 12V is probably as good a choice as any. I cannot think of a principle of electricity that can be shown at 120V that cannot be shown at 12V that would be suitable for high school students.

GFCI is a good idea for the receptacles supplying the lab cubicles.
 
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