Author Topic: RCD / GFCI finger test?  (Read 11803 times)

0 Members and 1 Guest are viewing this topic.

Offline SeanB

  • Super Contributor
  • ***
  • Posts: 15259
  • Country: za
Re: RCD / GFCI finger test?
« Reply #25 on: March 03, 2015, 07:34:45 pm »
Current limit into a short with a RCD will trip it in under 2 cycles, same as an imbalance. The caveat though is the RCD can take 5 cycles after that to start to open, and if it is a short then arcing will occur across the contact gap until the arc breaker bars finally cool it enough to stop. This can take 15-20 cycles of the mains until the contacts are far enough apart to stop arcing. So if you are a ground path and the current drawn by the DUT is 10A or so the device can take a half second to disconnect, even though with a 30mA test current it will be open circuit in under 5 cycles. Being shocked for the half second will definitely hurt you, and if the current path through you is to low impedance ( wet skin on both contact areas) there is a high current that will flow, in the order of a half amp or so worst case. Definitely enough to stop the heart in many cases, so YMMV here.

Kind of like stepping off the pavement into traffic without looking and walking across the road. You might get across a good number of times but eventually you will be a road pizza. Test the RCD with a 10k 1W resistor between line and ground, not your finger. Otherwise use the test button, which does exactly the same internally, using a resistor connected so as to create the imbalance current across the main sensor coil.
 

Offline N2IXK

  • Frequent Contributor
  • **
  • Posts: 696
  • Country: us
Re: RCD / GFCI finger test?
« Reply #26 on: March 03, 2015, 09:56:10 pm »
In US terminology, the 5 mA units are referred to as GFCIs, or Ground Fault Circuit Interrupters. These are the ones most US folks are familiar with, installed in kitchens, bathrooms, basements, garages, and outdoors. Intended for protection from electric shock in wet/damp locations.

The 30 mA units are referred to as GFPE, or Ground Fault Protection for Equipment. They are used to limit ground fault current in certain industrial applications, or in places where the 5 mA threshold of a GFCI is too low (buried heating cables for driveway de-icing is one). They are NOT to be relied on for shock protection for personnel.

Quote
This one here says they trip at 5mA.

http://www.thisoldhouse.com/toh/article/0,,249536,00.html

RCDes (GFCIes) come in different sensitivities, between 5 and 30mA are usual for for protection against electrocution but are considered to be in the  'High sensitivity' group.

Fixed household ones in Australia are typically 30mA
"My favorite programming language is...SOLDER!"--Robert A. Pease
 

Offline SteveyG

  • Supporter
  • ****
  • Posts: 731
  • Country: gb
Re: RCD / GFCI finger test?
« Reply #27 on: March 04, 2015, 09:24:38 am »
Kind of like stepping off the pavement into traffic without looking and walking across the road. You might get across a good number of times but eventually you will be a road pizza. Test the RCD with a 10k 1W resistor between line and ground, not your finger. Otherwise use the test button, which does exactly the same internally, using a resistor connected so as to create the imbalance current across the main sensor coil.

You don't test between line and ground, you should test between line and a neutral conductor on the other side of the sense coil. You shouldn't dump current to earth as if there is a faulty earth, you then potentially give someone in the residence a shock.

Offline Zero999

  • Super Contributor
  • ***
  • Posts: 13296
  • Country: gb
  • 0999
Re: RCD / GFCI finger test?
« Reply #28 on: March 04, 2015, 09:43:00 am »
RCDs also use a fusible resistor for the test function. If a normal resistor is used and the RCD fails to trip, the resistor would glow red hot, smoke and may catch fire.
 

Offline SeanB

  • Super Contributor
  • ***
  • Posts: 15259
  • Country: za
Re: RCD / GFCI finger test?
« Reply #29 on: March 04, 2015, 07:48:50 pm »
Kind of like stepping off the pavement into traffic without looking and walking across the road. You might get across a good number of times but eventually you will be a road pizza. Test the RCD with a 10k 1W resistor between line and ground, not your finger. Otherwise use the test button, which does exactly the same internally, using a resistor connected so as to create the imbalance current across the main sensor coil.

You don't test between line and ground, you should test between line and a neutral conductor on the other side of the sense coil. You shouldn't dump current to earth as if there is a faulty earth, you then potentially give someone in the residence a shock.

True, but a typical house normally has no access to the supply side neutral other than by the connection between neutral and PE at the point of supply. Thus the use of a resistor and a connection ( just for under a second) to PE. If it does not trip then you have either a faulty RCD, or a faulty outlet.

Here in S Africa electricians all use a test plug, that both shows correct wiring for the socket, but which also shows up a floating earth, before you start to increase the current till the RCD trips out. You start at 0mA here and go up in 10ma increments to 50mA, which allows you to find those that are both faulty (do not trip at 30mA) and those where there is existing current or the RCD is too sensitive (trips at under 20mA) to test each socket outlet.

For industrial use in water heating you get a special 3 phase unit with adjustable sensitivity, which will trip at a preset current between 50mA to around 500mA. This is used so that a faulty element does not just keep on arcing in the water if the case fails, but which will not trip for things like condensed water on the terminals.
 

Offline briselec

  • Regular Contributor
  • *
  • Posts: 94
  • Country: au
Re: RCD / GFCI finger test?
« Reply #30 on: March 05, 2015, 10:45:46 am »
I've tripped RCDs a few times and have never felt it.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf