Why is 110V safer than 220V? 3

[Speedy]

Problem goes like this ..........

We are currently using a heating tape (trace) on a flexible tubing. On the prototype machine we were using a 220v tape. We are going to production with the machine and because operators will handle the tubing I have safety concerns eventhough the tape itself is insulated, this in turn is covered with heatshrink.

I discussed this with our in-house electrician who suggested using a 110V tape with a Safety Isolation Transformer.

For my own 'peace of mind' I would like to know why this option is safer. I understand the princible of isolation in the transformer but do not appreciate how it could be safer. After all there is still 110V. I found the following below which explained about
muscle tightening etc.

Apprecaite any comments?



"110vac is a tad safer than 220vac because at 110vac, the
muscles still react to the voltage and go through a spasm which may release your
hold on the conductor. At 220vac, the muscles simply clamp in one direction
predominantly, and grip the conductor."

 

[dhwilliams]

I'm no expert in this field but I believe that power tools used on site use a 110V isolation transformer with an earthed centre-tap. This means that the highest "shock" voltage (with respect to ground) is only 55Vac. This is obviously safer than 110 or 220Vac.

Dave

 

[TheBlacksmith]

According to the US Navy, as little as 30 volts can kill - they specify a maximum of 28 volts on all controls and push buttons where one could make contact. Of course this is on a steel hulled ship with high humidity/dampness, so it may be conservative, but I personally have no desire to touch 55 or 110 volts.

Blacksmith

 

[dbaird]

Please review:

http://www.epanorama.net/links/safety.html#electrical

David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[Barry1961]

It takes a very small amount of current thru the heart to kill. Doubling the voltage with the same resistance will double the current. So given equal conditions 220v is more dangerous than 110v. In my personal experience 110v and 220v will both "grab" you, while 480v and over seems to "knock" you away.

Barry1961

 

[todcaesar]

With the isolation transformer you can touch the energized wire with one hand and a ground with the other and not be electrocuted because the ground is not in the loop with the isolated circuit. However, if you touch one one the conductors of the isolated circuit with one hand and the other wire with the other hand, you WILL die!

 

[ScottyUK]

todcaesar,

The isolation transformer described above by dhwilliams is isolated from the primary, but has a ground connection deliberately applied to the centre tap of the secondary winding. This ensures that it is not possible to receive a shock to earth greater than 55V.

Your second statement is incorrect. If you hold both ends of a 110V winding, you probably won't die. You will very probably receive an unpleasant shock. It is POSSIBLE that in particularly unfavourable circumstances you will suffer a fatal shock, but in the majority of cases this will not happen.

DC current has more probability of causing a 'gripping' effect, because it sends the muscle into spasm. The tiny electrical impulses in your brain are swamped by the shock current, and can not release the muscle. The effect is more serious when applied to the human heart, which is a group of powerful muscles and behaves in the same way as any muscle in your arm or leg.

 

[buzzp]

First off, if this is a heating system, the current drawn at 220Volts will be LOWER than the current drawn by the 110V system with the same wattage rating. Not the other way around as suggested.
So, current wise, the 220V system will be safer because less current is needed (current kills-as low as 80mA, depending on conditions). On the other hand, it is at a higher voltage, increasing the current one would experience if they were to grab both ends of the power supply.
Its my opinion that either are just as safe as the other. With a 110V system, the current will be higher necessitating larger conductors. WIth a 220V system, the conductors need to have insulation rated for 220V. However, generally conductors are insulated for 600V, so this is probably not a big deal. I would use whatever is most convenient and would not use an isolation transformer. Electricians should be accustomed to handling either voltage safely. Granted you would like to make it as safe as possible but there is a limit. Think about your household power. Do they use isolation transformers? Just my two cents. I have worked with so called electricians that wired up mostly 12VDC power on industrial vehicles. The one company I worked at, these guys were not electricians and had no clue. With that being said, maybe you do want to isolate the power source. This will only eliminate any shock hazards betweent the power source and the chassis ground. It will not help with shock hazards between the hot and neutral of the isolated power.

 

[dpc]

I agree with Barry1961. Everything else being equal, 110V should be somewhat safer than 220V because it will drive only half as much current through the same resistance compared with 220V.

While buzzp is correct that it is the current through the body that determines the risk of electrocution, it is not correct that the normal load current has anything to do with this. The current that is of concern is any current that is shunted through the body. 220V will drive twice as much current as 110V.

But, there is no question that 110V ac can be lethal under certain conditions.

If there is a safety concern, the heat tape should be fed through a GFCI.

 

[dspDad]

The typical skin impedance when dry is in the megohms, but when wet and/or abraded, is closer to 1000 ohms. This allows 100 mA or so at 110 and about twice that at 220.
It takes about 5 mA to cause tetany, and 5 mA hurts, and it takes about 20 mA to get 'glued' to the wire. The current density is higher in the hands than in the arms, torso and legs because the cross-sectional area is smaller, but the body is anything but a uniform conductor.

50 and 60 Hz are less dangerous than dc in part because of skin effect-the one that affects wiring. Below 0.5 inches, the current is significantly attenuated, but with dc, the current will be spread throughout the limb. This protects the heart a bit from ac, and the heart also sits in the poorly conductive lungs.

People who die from running hedgeclippers next to the swimming pool rarely die from cardiac arrest, they die from asphyxiation because the can't let go and they can't breathe. People who touch the 11kV power lines or get hit by lightning get cooked and have cardiac arrest which kills them.

Usually an isolation transformer has no connection to earth, so there is no current flow from either side to ground. Getting across the transformer is unlikely.

Getting shocked from the mains is almost always from one hand to the other hand or the feet, and the isolation transformer eliminates this possibility.

From a cost standpoint, isolation transformers are EXPENSIVE, and a gfi seems like a lot cheaper solution. From the description I don't see any unusual hazards that shouldn't be anticipated and avoided..like a warning that if the insulation is abraded, get it fixed. If you plan to run whatever this is in a bathtub, then an isolation transformer might make sense, but with that level of exposure, maybe a 12 volt strip heater or some other heating mechanism should be used.

 

[ScottyUK]

Another thought for Speedy:

Heating trace is available with an earthed screen. In the event of a failure in the insulation around the heating element, it is enclosed within an earthed braid, and will almost certainly trip the supply breaker. If used with an RCD (GFCI to the Yanks), the presence of the earthed braid in close proximity to the heating 'element' makes it very likely that the RCD will detect the fault in damp conditions as a short leakage path will exist.

 

[dpc]

NEC Article 427 requires ground-fault protection for heat tracing, although there are exceptions. The 2002 NEC added a requirement for a grounded conductive covering for all heat tape.

Of course, if you're dealing with a manufactured piece of equipment, the NEC requirements don't really apply.

 

[afterhrs]

Speedy,
In my opinion, your electrician is right to recommend 110 vac. Ground faults last a very short
time but the amperage can be substatiantal.
It doesnt matter to me what amperage you are running under normal operating condtions, but how much potential will be there in a ground fault situation.
The higher the voltage, the greater the potential.
Take a look at your average motor start circuit wiring that will carry 5 to 6 times the rated amperage for
a very short time on intial motor startup repeatedly and will probably outlast the motor it is wired to if the initial install was wired to code.
To me the most important thing is safety of the OPERATOR that will have his or her hands on your equipment during normal operation
For your "piece of mind," here are my recommendations for this particular situation.
1) 110 vac
2) heat trace with earth screen (Thanks
Scotty)
3) If possable, divide the trace up into 4 cicuits of
5 amps instead of 1 with 20 amps.
4) Use GFCI
5) Use fast acting fuses, not a breaker,
before the GFCI

Regards,
Afterhrs

 

[don01]

hi all
a small observation on my part.

In Japan the mains voltage is notionally 100V ac
In the USA it's notionally 110V ac
In Australia it's notionally 240Vac

Now people get killed every year by the 100V, the 110V, and the 240V. The are ALL lethal. The systems around them protect the people (RCDs, isolation txs, ELCB's earthing etc etc)

At the risk of sounding like a moralist, I cant see how you could call 100V or 55V safe. They may be less risky, but is that the game we are in?

Don

 

[Lakey]

Hi all,

I agree with dsDad.

It is my understanding that you may feel a shock from any voltage source >48V – under normal conditions (i.e. with dry hands). And any current >20mA can be fatal, but essentially the amount of shock will depend upon the resistance or impedance of ones skin.

That said 110V is safer that 220V, but it can still kill. I = V/R.

Best regards,

 

[Speedy]

Guys,

Thanks for all your help. It certainly seems clearer to me now.

We are going to use the RDC, Isolation 110V transformer, insulate the Trace Tape as much as possible with proper grounding etc. We cannot use the screened tape as this will reduce the flexibility of the piping.

I had concerns because the operator will grip the pipe within his fist..... I remember the old electrician advice of always touching a wire with the back of the hand first.

The environment is dry also, so no concerns in that regard.

Thanks again!
Speedy

 

[Ahr35181]

Speedy,

We use Raychem heating tape & use 30mA RCD's on all safe & hazardous area installations as per manufacturers advice:-

Raychem insists on the use of a 30 mA residual current
device to provide maximum safety and protection.
However, where there is a marked increase in nuisance
tripping, a maximum 300 mA residual current device may
be used.
For heating cables installed in a hazardous area, the use of
residual current devices is normally a condition of their
approval.

Alan

 

[pager]

listen to buzzp

 

[buzzp]

If all your considering is safety and not cost then 120 will be best due to a reduced shock hazard. This may be the case if the people installing are not qualified.
If cost is the real issue then 220 would work best since the current will be lower (smaller wire and controls). 220 will also allow higher rated heaters to be used vs. 120 volt heaters.
To me, 220, is the way to go unless you have people messing with electricity that should not be, even then 220 is just as likely to kill them as 110. Good luck.

 

[OperaHouse]

As Yoda would say, "There is safe, not safe. There is no safer." Any of you want to be on a witness stand after someone was injured and have you comments read out about how it was a little safer. You would be hung out to dry. Use a GFI period.