Excessive earth fault loop impedance

Martin Flint · Apr 17, 2019

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

[ElectriciansForums.net] Excessive earth fault loop impedance

telectrix · Apr 17, 2019

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

Charlie_ · Apr 17, 2019

What is the earthing arrangement?

plugsandsparks · Apr 17, 2019

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

Martin Flint · Apr 17, 2019

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.

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 · Apr 17, 2019

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

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 · Apr 17, 2019

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

telectrix · Apr 17, 2019

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

Andy-1960 · Apr 17, 2019

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.

Toneyz · Apr 18, 2019

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.

Martin Flint · Apr 24, 2019

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

Charlie_ · Apr 24, 2019

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

Martin Flint · Apr 24, 2019

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

NIge2103 · Nov 20, 2023

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.

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]

Excessive earth fault loop impedance

Martin Flint

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telectrix

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Charlie_

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plugsandsparks

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Martin Flint

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telectrix

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Ian1981

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telectrix

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Andy-1960

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Toneyz

Martin Flint

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Charlie_

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Martin Flint

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NIge2103

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