Excessive earth fault loop impedance

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Martin Flint

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• Apr 17, 2019

• #1

I work for a hotel chain and recently we’ve had our 5 yearly test they have 8 circuits from our LV panel with excessive loop impedance readings !! So apart from either introducing an rcd or dropping mcb ratings or type is there any other way of achieving satisfactory readings ??
Also if I have a fuseboard fed from another fuseboard ie a distribution circuit when doing a certificate is the (ze) on the furthest board recorded as a zs I think it’s called a zdb I’m just trying to understand there report as it doesn’t state anything just high loops
Can someone advice me please

 

telectrix

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• Apr 17, 2019

• #2

it sounds like maybe the Ze is excessive or a fault with main earthing. need far more info to make an accurate diagnosis.

 

Charlie_

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• Apr 17, 2019

• #3

What is the earthing arrangement?

 

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• Apr 17, 2019

• #4

Agree with tel. Need to trace loop impedance from origin and work out why its high.

 

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Martin Flint

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• Apr 17, 2019

• #5

telectrix said:

it sounds like maybe the Ze is excessive or a fault with main earthing. need far more info to make an accurate diagnosis.

Click to expand...

I’m going to the hotel tomorrow to have a look will provide more info then but if it is the ze wouldn’t all the circuits have a high reading ?
Also regarding zdb is the ze from another dB recorded as a zs reading ?
Many thanks for your reply

 

telectrix

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• Apr 17, 2019

• #6

Martin Flint said:

I’m going to the hotel tomorrow to have a look will provide more info then but if it is the ze wouldn’t all the circuits have a high reading ?
Also regarding zdb is the ze from another dB recorded as a zs reading ?
Many thanks for your reply

Click to expand...

yes. your Ze is the EFLI at source of installation.all readings downstream ( after main earth) are Zs. ( sometimes referee to as Zdb at dis. boards. ) these Zs readings are taken with bonding connected, as opposed to the Ze where bonding is not.

 

Ian1981

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• Apr 17, 2019

• #7

Presuming that the LV panel contains mccb’s then It could be that either the maximum zs that the inspector has allowed for the device is incorrect and it actually complies(manufacturer’s data is required) or that the original install , incorrect maximum zs values for the ocpd was incorrectly assumed to comply when in fact it never did.

Or as others have said the Ze is on the high side.
Some results/values of the excessive zs values might help

 

Last edited: Apr 17, 2019

telectrix

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• Apr 17, 2019

• #8

another thing springs to mind. maybe testing at no-trip and the MCBs are corrupting the readings due to internal components.

 

Andy-1960

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• Apr 17, 2019

• #9

Another thought, how close is the main DB to the supply transformer and has the tester used a MF meter that is suitable with a high current test facility?
Standard domestic type MF meters can give incorrect Zs readings if close to the supply transformer.

 

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Toneyz

• Apr 18, 2019

• #10

Do you have a previous or better still initial report/certificate? as for your question other than what you posted to overcome this would be the increase of conductor size/additional earth.

 

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Martin Flint

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• Apr 24, 2019

• #11

Hi sorry for delay I’ve been to the job and they are protected by Berlin gerun BS100N Mccb I have 3 phase 100a and single phase 63a and 32a they are saying max zs is 0.10 for these can anyone confirm or tell me what the max zs is for these many thanks

 

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• Apr 24, 2019

• #12

100 = .24
63. = .38. (.46/5s)
32. = .48. (.77/5s)

 

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• Apr 24, 2019

• #13

Many thanks o did s bit of digging and worked out 0-24 so I knew they all comply but thanks for your help

 

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• Nov 20, 2023

• #14

Andy-1960 said:

Another thought, how close is the main DB to the supply transformer and has the tester used a MF meter that is suitable with a high current test facility?
Standard domestic type MF meters can give incorrect Zs readings if close to the supply transformer.

Click to expand...

Andy, I found this post of yours from four years ago. We (I am the QS) are having some issues with larger (400A -1250A) MCCBs/ACBs having excessive Ze readings. What do you mean when you say MF meter? Is this like a Megger 1711 or something like? [email protected]