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Discuss rcd testing in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

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hawk666

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hi guys , all i want to know really is why we test at 0 degrees and 180 degress of the sine wave when testing the rcd , cheers guys:)
 
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Guest123

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  • #2
Hey.

As we are on an AC system the current alternates between the positive and negative sides of the sine wave 50 times every second hence 50Hz frequency.

This is why the RCD is tested 0 and 180 to make sure it will operate at both positive and negative sides of the cycle which are 180 degrees apart.

Hope this makes sense.:D
 
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Dela Boy

  • Thread Starter Thread Starter
  • #4
Given that the time between 0º and 180º is 10ms, i have to question the point of the 1 I∆n 180º test? If your RCBO trips at 290ms your still going to replace it aren't you. At 5 I∆n it makes more sense. Still looking at how an RCD operates i can't see how a device tested to 0º successfully would then fail at 180º.

I don't have the answers just the questions.
 
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Spazz

  • Thread Starter Thread Starter
  • #5
Just remember that RCDs are fault protection
It protects 2 situations:
Human coming into contact
Fault between Line/neutral and Earth

With the human contact - the resistance of the body is over 150mA (with the average human) therefore tripping in 40ms

With a fault between L/N and Earth there could be a very high resistance of 30mA (e.g. insulation of cables) but not high enough to maintain a high enough resistance to prevent any short between L/N and Earth.
These will trip 300ms (or 200ms).

Yes for direct shorts it will trip in 40ms
 
Old RCD testers only test 0 deg. There isnt a huge amount of point in a 180 test.

Nick, I'm struggling to understand your explanation, I think your confusing your terminology, the resistance of human body is roughly 1000 ohms if sweaty etc. this would allow about 10ma to flow, which proably wouldn't trip the RCD. It would take 30ma to trip it in 300ms.
 
Just remember that RCDs are fault protection
It protects 2 situations:
Human coming into contact
Fault between Line/neutral and Earth

With the human contact - the resistance of the body is over 150mA (with the average human) therefore tripping in 40ms

With a fault between L/N and Earth there could be a very high resistance of 30mA (e.g. insulation of cables) but not high enough to maintain a high enough resistance to prevent any short between L/N and Earth.
These will trip 300ms (or 200ms).

Yes for direct shorts it will trip in 40ms
Isn't resistance measured in ohms? Or have I missed an update somewhere?
Sorry mate your entire post makes no sense to me
 
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SirKit Breaker

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  • #8
I will clear up a point or two here. Firstly, Lenny has explained all there is to say. I will add the reasons why we use a 30mA RCD, and have a trip time of 40ms at 5 times its rating.

30mA is sufficient current to kill you by causing your muscles to go into spasm. If your chest muscles spasm, this will stop you breathing, no breathing, you die.
But it needs to disconnect within 40ms, you can hold your breath for longer than 40ms, and once it has tripped, normal service will be resumed, and you will not die.

Cheers...........Howard
 

ackbarthestar

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Mentor
Arms
Although I agree with SirKit Breaker, I think there is a little more to add.

It has been historically shown that 95% of the population is able to withstand an electric current of 30mA for a given period of time, precisely unknown. but a lot less for larger currents so for shock protection,
If we assume that the average heart rate is between 60 - 120 beats per minute ( 1-2 beats per sec), and that the most sensitive part of the heart beat cycle, when the muscles contract to pump the blood, is between 10 - 20% of the heart beat. Then as a rough approximation, and taking the worst case (120) there would be a time interval of 50mS to 100mS.

So the worst possible scenario would be that the current flow lasted for a time period greater than the sensitive period causing, most probably, heart muscle fibrillation.
By reducing the length of time that a fault current flowed to below the average heart beat's sensitive time period would be the obvious answer. Hence 40mS. This would, IMO reduce the possibility of heart muscle fibrillation.

Testing a RCD in both the positive and negative half cycles is to ensure that the fault is detected in both half cycles. Some older RCD sometimes wouldn't trip on the negative half cycle.
 
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albnegru

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  • #11
an RCD must be tested at 0 and 180 to ensure
1. correct operation on both halves of the AC cycle
2. correct operation when used on DC supplies
3. the integral test button operates the RCD smoothly
4. correct polarity of the supply
 

telectrix

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Mentor
Arms
Esteemed
post what you think. you learn better that way. once we have your answer, then we can advise if you are right or wrong, and offer advice.
 
an RCD must be tested at 0 and 180 to ensure
1. correct operation on both halves of the AC cycle
2. correct operation when used on DC supplies
3. the integral test button operates the RCD smoothly
4. correct polarity of the supply
If you be bothered to read this thread you will find your answer easy.
 
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albnegru

  • Thread Starter Thread Starter
  • #14
i'm not from UK.i don't know too much english.it's very dificult for me to learn in english,for this i need help sometimes
 
With all due respect, you are going to find a level 3 course very, very tough if you can't revise through an English forum thread to find out the answer to your question!
 

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