Electrical Safety

[Ken I]

In my line of work I do a lot of electrical work - mostly 220/380 plus the odd HT job.

I have had a number of jolts over the years and sort of accepted it as an occupational hazzard - which I'll admit is a really dumb philosophy that I have abandoned.

About 6 years ago I recieved a bad jolt - 220V from one hand to the other - through the chest - this is particularly dangerous. This was my second bad shock in a month and were rated on my personal top ten list as #2 & #3 worst ever.

This made me take stock - one day it will be your last shock.

So from then on I religeously assume all circuits are live and do not touch without first checking with a meter. (I have never managed to shock myself working on a known "live" circuit - its always the assumption that a circuit is "dead" that gets you.)

The results - I have not received a single shock in over 6 years whereas prior to that I was probably averaging 3 or 4 per year.

The lesson:

You have to consiously work at safety.

Don't ever get complacent or blasé about safety.

Treat all electrical installations as "live" - no exceptions.

Ken

 

[kherseth]

Very true, and if you have to work on a live system, keep one hand in the rear pocket at all time. Those hand-to-hand shocks gives a cardiac arrest. In most cases, short shocks is no problem as the heart starts again on itself. Longer exposure on the other hand...
Really weird how fatigued one feels after a a real shocker.

 

[Stan]

Good advice at any time. As we were told, "Electricity is a good friend and a bad enemy".

 

[JorgensenSteam]

I was working on an outdoor sign years ago, and it was fed from overhead wires.
A tree had grown up under the wires an lifted them somewhat, but not to the point of them being tight.

I turned off what I thought was the circuit breaker serving the sign, climbed up an aluminum ladder, used a multimeter to measure from the black conductor to the white conductor (hot to neutral), and got a reading of zero.

I proceeded to grab the black conductor to disconnect it, and got zapped big time, but not killed.

As it turns out, the copper conductor for the neutral broke inside the insulation where the tree lifted the wire, but the insulation did not break. I failed to measure from the hot to ground, but if the sign was not well grounded, that could have given a false reading also.

Now days, I turn off what I think is the correct breaker, and then (using insulated handle pliers) short the hot to the neutral, and then both to the ground wire, and then all that to the metal frame of the device.

Then I make sure the device does not have a dual feed, such as a separate control and power circuit.

You have to be very careful working on equipment, whether you think it is engergized or not.

A split wired receptacle got me one time, as did a split wired circuit with a light switch and receptacle.
Split wired receptacles are fed from two separate circuit breakers.

Pat J

 

[steamer]

My brother is an electrician and a good one with about 30 guys working for him.

He lost his right hand man to a 440V panel. He doesn't really know what happened, but he was in a live panel. Nuff said.

It'll get ya....every time.

Dave

 

[boatmadman]

Here is a good demo of when electricity gets you:

http://www.youtube.com/verify_age?next_url=http://www.youtube.com/watch?v=m3_ear4k-Jg

In this case, a breaker was being put back on the busbars already in the "on" position.

Ian

 

[Ken I]

I had a similar sized board do exactly that due to a hot joint on the incoming riser from a 1MVA transformer, the arc jumped an open bus coupler and fed 2MVA of unprotected (LT side) power into the fault - this is a fault current in the order of 40kA which probably lasted 0.2 seconds before the H.T. tripped at the supplier's sub-station.

The panel was so hot we had to wait 40 minutes for it to cool down before we could start repairs, the entire inside of the panel looked like someone had clumsilly dragged an arc welder all over it. There was a fist sized hole burned through a 4mm stainless plate.

Almost all the panel doors were blown open by the blast.

Fortunately there was no one in the vicinity at the time.

Many years earlier while commissioning the exact same panel, the local electrical inspector (who had to sign off the work) said to me "there's not enough clearance between those busbars." I said "Where ?" and he pointed with a long thin screwdriver he habitually used for such purposes but unfortunately he shorted the bars and it blew up in our faces - we looked like black and white minstrels (wearing safety glasses) but no real harm done.

Needless to say the inspector was embarrased to the nth degree and was very easy to deal with after that.

Ken

 

[black85vette]

I have one of these and love it:

http://www.familyhandyman.com/DIY-P...cal-Tools/how-to-use-cheap-electrical-testers

Detects AC power without making contact. Requires no ground or neutral. Works through insulation. You can find a hot wire in a bundle of wires or stick it into an outlet to see if it is hot. Has a light and sound indicator. Very small and you can clip it into a pocket. Can be used one handed unlike a multi meter.

 

[fcheslop]

A simple rule that was drummed into me as an electrical apprentice was always ISOLATE AND CHECK you might not get a second chance .Work safe and never believe whats marked on a distribution board.You cannot see it or smell it but you can really feel it and it Hertz.
best wishes Frazer

 

[maverick]

Pat,
Thanks for jogging my brain. When I wired my shop I ran boxes down both sides, installed 2 receptacles each and pulled 2 separate
circuits alternating each receptacle (trying to balance the load). If only one breaker is off there is still power to the adjacent receptacle.
I just went out and labeled the boxes with both circuit numbers and a warning to turn them both off for service. You may have helped keep me safe and
I need all the help I can get.

Mike

 

[JorgensenSteam]

Mike-

That is a good idea.

I am also adding e-stop switches, each with its own contactor, to my lathe and mill, which did not originally come with these devices.

I also heard that those electrical testers don't necessarily work in all cases, for instance there are systems called "corner grounded deltas", and if you have that system (in a commercial building), then you can have either 240 volt 3-phase, or 440 volt 3-phase, and one phase is grounded, so the tester will not catch it, but it is still an energized circuit, and very dangerous.

Pat J

 

[black85vette]

The electric device that I thought was the poorest design I ever saw was a timer for a heater with a 220 volt line but the timer only had a single contact in it and instructions showed it installed and just interrupting one leg of the 220 v circuit. Sure it shut down the heater it controlled but left the heater wire with 110 volts on it at all times! I showed him how to install a dual contact relay to fix it. That was a serious accident just looking for an opportunity.

 

[lordedmond]

I am an Electrician yes safety is very important for your life , when I was in work my electrician had a pact with never trusted each other to isolate we always tested each others isolation and no offence was inferred ,hence both worked with safety


On another note when talking about electrical systems ( this is a world wide board ) please state as to the country of origin of the poster , e.g. TP&N may not mean anything to others ( Triple Phase & neutral )

just for completeness I live in the UK and a single pole breaker would be fine if it where in the phase the equipment would be at zero volts ( neutral is earthed at the transformer and at the consumer cutout )

ie we run 240 volt single phase ( domestic ) with the hot wire 240 above earth , 440 volts between phases again 240 to earth

Be safe and test your tester before and after use ( test a known live source ) you never know your tester could fail at the wrong time


Stuart

 

[JorgensenSteam]

Most residential homes in the US are fed from a single phase 120/240 volt transformer, which has two lines and a neutral. The neutral is grounded at the transformer and at the electrical service entrance location.

The neutral and ground bus are bonded together at the electrical service entrance panel only, but not any other downstream panelboards (which would create a dangerous situation).

For most residential use, line-to-line voltage is 240 volts, single phase, and line-to-neutral or line-to-ground voltage is 120 volts., single phase.

It is generally a good idea to use separate junction boxes, or junction boxes with dividers when two separate circuits each fed from a different line (L1 and L2) are fed into the same box, such as when you have two receptacles side-by-side fed from differnent 120 volt circuit breakers, since the voltage between the adjacent bare screws on the sides of these two receptacles is 240 volts.

Many people confuse the neutral wire, which is a current carrying conductor with the ground wire, which only carries current during fault conditions. Ground wires are often not insulated in residential wiring after the outer insulation jacket is stripped back, and using a ground wire as a neutral creates a dangerous situation, since anyone or anything that contacts the bare ground wire that is being used as a neutral can receive the full voltage and current of that circuit.

A 120 volt ground interrupting receptacle can be installed at the beginning of a circuit, and if connected properly, it can protect all downstream 120 volt receptacles on the same circuit. I prefer the 20 ampere GFCI receptacles since many devices in the shop need that much capacity.

I have used GFCI type circuit breakers, but they are not very reliable (in my experience).

The European systems save a lot of copper by using the higher voltages, since doubling the voltage reduces the current by 1/2 in a single phase circuit.

Steel mills and refineries in this area generally bring in 161 KV circuits, and then break those down into 35 KV distribution circuits, which are further reduced to 480 volts, etc. using unit substations with oil or dry type transformers.

Large shipping facilities use 23 KV distribution here, and many large plants use 12.47 KV distribution systems with outdoor metal enclosed switchgear.

In the commercial/industrial world in the US, most are standardizing on the 480Y/277 volt systems, with 480 volts feeding motor loads, and 277 volts feeding lighting circuits, and then 208Y/120 volt dry-type step down transformers for small loads, with the dry-type being a delta-wye with the secondary neutral being connected to the building steel.

Rural utility cooperatives are beginning to standardize on the modern transformer voltages and connections, but many still use obsolete and dangerous connections such as the corner grounded delta (either 240 volt, 3-phase, or 480 volt, 3-phase). Three phase motors used in remote rural areas used for intermitant loads such as well pumps are often fed from an open-delta transformer connection, which derives 3-phases from only two single-phase transformers.

Some obsolete rural systems use three 240 volt single-phase transformers, with one transformer being oversized and having a centertap to provide 120 volts. Mixing up the conductors in this type system is very dangerous since you can get a whole lot more than 120 volts with the wrong connection.

There are a large number of transformer connection types (delta-wye, wye-wye, grounded, non-grounded, high resistance grounded) and each has its own particular characteristics. You cannot safely work on an electrical system without a complete knowledge of transformer connections.

Another common mistake I see throughout the industry is to use the wrong nomenclature to describe a circuit voltage.
A 120/240 system is a single phase residential-type connection. A 208Y/120 volt system is a 3-phase system, and totally different from a 120/240 volt system, and yet in this region, the nomenclature 120/240 is commonly used to describe both systems.

The Code in the US allows the use of metallic conduit instead of a grounding conductor. This practice is dangerous, since you will almost always have either a discontinuity in a part of the conduit system, or the resistance of the conduit will be so high that during a fault condition, the circuit breaker will never trip, and the equipment frame can be energized and extremely hazardous. A separate equipment grounding conductor should be used with every circuit.

Aluminum wiring should be avoided unless you are a utility company, and use it in an outdoors application, and terminate it with the correct compression lugs and de-oxidizing compound. I have seen high quality indoor aluminum wiring overheat just because the terminations would not stay tight, or the terminations were not cleaned and deoxidized, or the terminations built up high resistance (causing high heat) due to oxidation of the aluminum.

That is the story on the most commonly used US systems.

Pat J