Live Or Dead Testing - A Question For Members

[steptoe]

the test button simulates an earth fault by by-passing one of the transformer windings

Id still like to know where this has came from though.

 

[steptoe]

les,

The test button on an "RCD" only proves the mechanical operation of the device.

There is no more to say.

That is correct.

so why have you said it creates an imbalance?

 

[says-les]

Steptoe,

An RCD has a current balance transformer inside. The two windings carry line and neutral current respectively. Without a fault these currents are equal and the trani is balanced. In event of earth leakage the current in neutral winding is less than in the line winding so the trani has unbalance and induces current into the trip coil wound on the trani core. This is connected across the trip coil of the breaker and causes a trip current to flow into the trip coil. The breaker trips within 40mS

 

[says-les]

Sidewinder,

i'll put it another way.

we need to make sure we use equipment like fuses or CBs that has rated fault capacity greater than the Ipssc measured at that point. agreed?

if this is not the case then the fuse or CB is not able to clear such Ipscc safely without damaging itself and its surroundings. Agreed? so what is not right about this?

Next, I agree that the test button on an RCD only checks the mechanical operation of the breaker.

What is not right about anything relating to this I have said?

 

[steptoe]

Steptoe,

An RCD has a current balance transformer inside. The two windings carry line and neutral current respectively. Without a fault these currents are equal and the trani is balanced. In event of earth leakage the current in neutral winding is less than in the line winding so the trani has unbalance and induces current into the trip coil wound on the trani core. This is connected across the trip coil of the breaker and causes a trip current to flow into the trip coil. The breaker trips within 40mS

Sidewinder,

i'll put it another way.

we need to make sure we use equipment like fuses or CBs that has rated fault capacity greater than the Ipssc measured at that point. agreed?

if this is not the case then the fuse or CB is not able to clear such Ipscc safely without damaging itself and its surroundings. Agreed? so what is not right about this?

Next, I agree that the test button on an RCD only checks the mechanical operation of the breaker.

What is not right about anything relating to this I have said?

the post above that you made.

:|

 

[says-les]

which post do you mean?

 

[Technician]

Answering Says Les, the larger fault current must be considered

when selecting the protective device.  This will determine the choice

of this device since its rupture capacity should exceed the larger

fault current.

I have in my possession a minor works cert. for some work carried out

at my house some years ago.  The fault current is given as 2.7 kA.

This, at the time when the fuses were the old BS3036.

 

[says-les]

Breaking capacity of BS 3036 is only 2kA, so Ipscc should be less than this or this type of fuse could be damaged trying to clear a dead short close to it. I see what you mean and I was just explaining why if any of the guys were not sure. Thanks for that sir.

 

[apprentice87]

Hi Says-Les, Just thought i would point out that you appear to be collecting scoobs as an impressive rate!!! We must all like you here!!! Got to say that this is one of the best posts for a while!!!

john..

 

[says-les]

thanks John. truth is I taught Iee Regs since 14th Editionand would never claim I know them thoroughly. I at one time had to teach maintenance sparks and they had never studied the Regs. It was good but challenging. the guy who updated me on 17th and Inspection & Testing was a student of mine but was a brilliant lecturer. In that same church I have been referring to I met with m class mates in order to practise testing as well as discuss past papers but college stopped being connected with it because of insurance. The written exam has been reduced to 1.5 hours from 2.5 hrs and we were NOT allowed to do live Zs testing except for Ze and we had to wear eye protection for theive part. when I posted this original subject less than a week ago very no member agreed wirh this nor the RCD/Zs issue. thanks for your input, John.

 

[apprentice87]

Hi Les, I cannot see why anyone would think it wrong to provide ADS by means of an RCD if Zs is too high for an overcurrent device to be used, as the regs SPECIFICALLY say that you can..  Ok, it might not be ideal, you know, with total reliance being placed on an RCD in case of an earth fault [as opposed to an ocpd AND an rcd, an rcd not being the most reliable device] but what about if you were to have TWO rcd's in series, like "redundant protection" as it were.... Besides, if you were not permitted to use an rcd if Zs was too high, then every TT system in the country would not comply with the regs..... hmmmm..... john!!

 

[says-les]

well, i hope we are both right but we are def in minority on the forum. i phoned a spark I know and he at first disagreed till I told him My and his lecturer had given me assurance about the issue. he said well if he said it then I agree!

 

[says-les]

please, Guys, remember I inherited this installation which was NOT wired by electricians 20 years ago. no documents. no EIC, any drwgs they have are not as fitted and useless. a small contractor is preparing an EICR having spent 3 days t a cost of £650. i have been pushng to have this done as I am retired but qualified to latest C&G qualifications

 

[says-les]

for Steptoe, please go to

http://www.tlc-direct.co.uk/Book/5.9.2.htm

and read about RCDs

 

[professional]

I have to disagree with you, Professional. Remember this is not a new job that I or anyone has just installed, but rather it is a 20 year old building that I have taken an interest in as member of the organisation that belongs to the building. Now, in order to add additional protection I added pods to the C-Type breakers. Now a professional engineer agrees with me that as soon as that is done, i.e. an RCD fitted then any/all such circuits need not rely on the CB to clear an earth fault. Now in any job if the correct rating of protective device matches the current rating of the protected cable in that circuit, then no one need be concerned if that overcurrent protective device will protect the circuit from short circuit faults. You are, or one spark replied that he is concerned about testing the L-N loop!!!! Nowhere does any Reg say that this L-N loop has to be tested. It will protect against short circuit faults if it has been rated to suit the size of cable, taking rating factors into account.

The regs do require additional protection now for certain circuits as you know such as where cables are less than 50mm from the surface of plastered walls, and in every socket-outlet circuit, and any type of circuit in bathrooms and other special locations. In these cases this is classed as additional protection, and only the RCD test need be applied but the Schedule of Test Results must be filled in for Zs and R1+R2 and Ze. If Zs is higher than normal for any reason and not your design of installation then fitting an RCD is additional protection BY CHOICE - nout wrong with that, and you will have read that a 30mA RCD is effective with a Zs of up to 1667 Ohms.

Cheers

Absolute rubbish - the use of an RCD does not obviate the requirement for basic protection - the RCD is there as additional protection in the event that the provision for basic protection fails!

 

[says-les]

if that is rubbish then why is an RCD to be fitted now for socket-outlets and orher specified loacations like barriers. ypu have misinterpreted the word additional. I regard it more as closer not in case the Fuse or CB fails!!

 

[says-les]

proffesional. you are taking on the role of authority in this. not even the IET will comment any more and they wrote BS 7671! Did you know that? it is down to the court lawyers to fight any dispute after a death or fire blamed on this.

 

[steptoe]

for Steptoe, please go tohttp://www.tlc-direct.co.uk/Book/5.9.2.htmand read about RCDs

Why?

I know that not all RCDs have what you call a neutral imbalance facility on the test button,

 

[says-les]

since we agree that it only tests mechanical parts can work it must be done by creating inbalance in the trani as per the reference site I posted. sid you read and study it?

 

[Technician]

Fault Protection;  Protection against electric shock under single fault conditions.

Provided by automatic disconnection in the event of a fault. (Reg 411.1)

Additional protection; only RCD's set to 30mA and operating within 40 mS at 

150mA comply.

Additional protection MEANS additional to FAULT protection and to BASIC

protection. (reg 410.3.2)

Reg 415.1.1 makes mention of careless users.