loop impedence test

[picklesmartyn]

Hi guys I was wondering if someone can help me with a question on my 2394 paper it says:

Why is it necessary to carry out an earth fault loop impedence test before carrying out an RCD test

My answer:

Because an earth fault loop impedance test will prove that there is a fault path to earth and without a fault path to earth an RCD cannot operate as they only operate when a live to earth or neutral to earth fault occurs.

If you could tell me if im correct or not in what im saying and if im correct do i need to elaborate a bit more

Thanks

 

[Richard Burns]

I would say that an EFLI test would be required prior to an RCD test because you need to ensure that the installation complies with ADS before powering the installation to test the RCD.
An RCD could work without a path to earth as the current only needs to not go through line or neutral, though a path to earth is the most likely, but you could have an earth path through a body to the ground and an RCD would still operate, think RCD protection on class II equipment.
However your approach would seem partially valid.

 

[picklesmartyn]

but where you said you need to make sure it complies with ADS which would be the zs before powering the installation to test the RCD your already powering up the installation to do the zs. I dont want to offend but ive noticed that the questions are very particular in what they want to see in the answer of the 2394 exam you know

 

[telectrix]

It's maybe a silly question, as you should test the rcd with nothing connected.so a efli test on the circuit then means you have to connect the circuit to the rcd, then disconnect it to do the rcd test.

 

[spark 68]

I answered this the other day on another thread.

You need to know that the ELI is low enough for the Idn rating of the RCD you are fitting to ensure the touch voltage does not rise above the specified volatage, eg. Ut 50V, in BS7671 there is a table (41.5) and formula that gives the max Zs allowed for a particular RCD Idn rating.

RA X Idn <= 50V
or Zs X Idn <= 50V

This formula can easily be transposed to give the values you require

 

[D Skelton]

I answered this the other day on another thread.

You need to know that the ELI is low enough for the Idn rating of the RCD you are fitting to ensure the touch voltage does not rise above the specified volatage, eg. Ut 50V, in BS7671 there is a table (41.5) and formula that gives the max Zs allowed for a particular RCD Idn rating.

RA X Idn <= 50V
or Zs X Idn <= 50V

This formula can easily be transposed to give the values you require

That formula only applies to TT systems buddy, on TN systems it is:

Zs X Ia <= Uo

A maximum Zs of 7667ohms being allowed on a 30mA RCD as opposed to 1667ohms on a TT.

As far as the rest of your answer goes, spot on I'd say

 

[spark 68]

Not quite,

If you take the 50V Ut and a 30mA Idn for example and transpose the equation.

Ut/ Idn = max Zs

50V / 0.03 = max Zs

50/0.03 = 1666.66666..... or 1667 ohms, of course with a Zs this high the tester would inhibit the test @ 5 X Idn .

Edit: the tester would not be able to complete the RCD trip test @ X1 with a Zs of 7667 ohms

 

[D Skelton]

Not quite,

If you take the 50V Ut and a 30mA Idn for example and transpose the equation.

Ut/ Idn = max Zs

50V / 0.03 = max Zs

50/0.03 = 1666.66666..... or 1667 ohms, of course with a Zs this high the tester would inhibit the test @ 5 X Idn .

Edit: the tester would not be able to complete the RCD trip test @ X1 with a Zs of 7667 ohms

I don't understand?

Are you saying that Zs x Idn <= 50V applies to TN installations as well as TT installations?

 

[spark 68]

The aim is to keep the CPC Ut and any exposed earthed metalwork below 50V regardless of the earthing system.

 

[D Skelton]

Yes but we're talking about maximum permitted Zs values in order to achieve required disconnection times, not touch voltage between parts.

The maximum permitted Zs allowed on a TN system protected by a 30mA RCD is 7667 ohms. Table 41.5 applies to TT systems.

We digress however lol. The point is that the OP needs to be able to determine that the external earth fault loop impedence is low enough to allow suitable operation of the RCD before testing the RCD.

 

[spark 68]

The max 1667 ohms for an Idn of 30mA applies to all earthing systems regardless to keep any Ut below 50V, by virtue of a low Ze/Zs TN systems generally comply by being low enough to satisfy this with an OCPD and rely on speed of operation rather than an RCD, think what the reasons for supplementary bonding is for on a TN system.

 

[D Skelton]

411.4.5 mate. Zs x Ia <= Uo.

That said, thinking about it, we may both be completely wrong anyway. Could the answer be related to unwanted tripping of the RCD whilst carrying out an external earth fault loop impedance test?!

I'm thinking textbook answers here! lol

 

[spark 68]

411.4.5 mate. Zs x Ia <= Uo.

That said, thinking about it, we may both be completely wrong anyway. Could the answer be related to unwanted tripping of the RCD whilst carrying out an external earth fault loop impedance test?!

I'm thinking textbook answers here! lol

That is for an OCPD where speed of operation is what counts (0.2s and 0.4s in table 41.1) and the Zs is low enough to achieve those speeds.

Back to RCDs your MFT will inhibit the test if the CPC rises above 50V, some MFTs allow Ut to be set @ 25V

About 334 ohms max for the 5 X Idn test for a 30mA device and a Ut of 50V

 

[D Skelton]

That is for an OCPD where speed of operation is what counts (0.2s and 0.4s in table 41.1) and the Zs is low enough to achieve those speeds.

Back to RCDs your MFT will inhibit the test if the CPC rises above 50V, some MFTs allow Ut to be set @ 25V

About 334 ohms max for the 5 X Idn test for a 30mA device and a Ut of 50V

Earth fault loop impedance and disconnection times are intrinsically linked. That is why I am putting forward the significance of 411.4.5. I can't see why you would carry out an EFLI test before an RCD test in order to ensure that voltage won't rise higher than Ut.

Like I said, I reckon we're both wrong anyway so it is potentially a mute point with regards to the OP's question.

 

[spark 68]

This is what Howard wrote in reply to me on HHDs thread about why his RCD would not test.

Re: Cannot get RCD test to work?

Originally Posted by spark 68
In addition to Howards post above ^^^^ Have you tested the ELI prior to fitting the RCD ?, because if the earth loop is high resistance/non-existent, the tester will inhibit the test if the CPC rises above a pre-set touch voltage, usually set @ 50V, some testers allow this to be set @ 25V, so check this too.



I forgot that bit, thanks. prior to carrying out a RCD test, the earth fault loop impedance value must be confirmed as compliant. That is an answer in the 2394/5 exam as well.

Cheers............Howard
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