earth fault loop impedance

[Gary Tollison]

Hello

I'm confused about EFLI which I am guessing should be relatively straight-forward.

I can't get my tired brain to figure out how to calculate the value for EFLI on system-side circuits?

eg:

what should the actual calculated value be for a 40A circuit, and how do I get that figure?

any help would be great!

Thanks in advance!

Gary

 

[westward10]

What do you mean by system side circuits.

 

[SparkyChick]

Zs = Ze + (R1+R2) where Ze is the external earth fault loop impedance measured at the origin of the installation, R1 is the continuity of the line conductor and R2 is the continuity of the CPC.

Zs will often vary as a result of parallel earth paths, resistance in connections.

In terms of what the calculated value will be, the bigger the cable, the lower you would expect Zs to be, the longer the cable, the higher you would expect Zs to be.

But what you're aiming for is the maximum permitted Zs to ensure the protective devices operate in the required disconnection times.

So for a 40A Type B breaker to BS EN 60898 you would want to see a Zs no higher than 0.88 ohms (this is the temperature compensated value from the OSG). This allows the circuit to achieve a fault current of (I=V/R => 230/0.88) 261A, this is well above the current required for 0.1 to 5 second disconnection time.

 

[Pete999]

Hello

I'm confused about EFLI which I am guessing should be relatively straight-forward.

I can't get my tired brain to figure out how to calculate the value for EFLI on system-side circuits?

eg:

what should the actual calculated value be for a 40A circuit, and how do I get that figure?

any help would be great!

Thanks in advance!

Gary

Zs=Ze+(R1+R2) So you will need to measure Ze measure R1 nd R2

 

[Ian1981]

230 v x cmim =218.5
218.5/ minimum current in amps given for device in appendix 3 to trip between 0.1-5 seconds for mcbs
= max zs at 70 degrees
For 40amp type b is 40x 5=200 amps
218.5 volts/ 200= 1.09 ohms

For type c 40x10=400amps
218.5/ 400=0.54 ohms

 

[westward10]

That will have finished him off.

 

[westward10]

Too much theory for me, get more info first before we all go Einstein.

 

[telectrix]

Ein Stein.

 

[Gary Tollison]

that hurt my brain!

The reason I am asking is I am having a go at a practice theory question, which is:

1. The following measured values of loop impedance were recorded for six circuits in an installation. Each circuit is protected by a BS EN 60898 type B cb. Referring to the maximum tabulated values given, show by calculation if these measured valus are acceptable.

2. For any circuit that does not comply, suggest a method of overcoming the situation.

Measured impedance values in ohms:

CT1 CT2 CT3 CT4 CT5 CT6
40A 32A 32A 20A 16A 6A
0.5 1.25 0.96 1.62 2.0 7.3

Maximum tabulated values in ohms (BS7671)
1.15 1.44 1.44 2.3 2.87 7.67


And I am confused!

I have beer in my fridge...maybe I should have one!

 

[telectrix]

i presume it wants you to apply temp correction and Ci factors to get a revised max. value for each.

 

[Gary Tollison]

I makes no mention of either

 

[westward10]

The give away is where it says "measured values". Those values of Zs you quote are not as stated in the current Edition of BS7671 they are from an old version.

 

[telectrix]

if you look at the 32A, the value measured complies as it stands ( 1.25 < 1.44 ) but once you correct the 1.44 ( x. 0.8 x 0.95 = 1.0944 ) so 1.25 then does not comply

 

[westward10]

I suspect they want the "rule of thumb" calculation which seems dumb if this is an exam question. The rule of thumb being your measured readings should not exceed 80% of the tabulated value in BS7671.

 

[Ian1981]

that hurt my brain!

The reason I am asking is I am having a go at a practice theory question, which is:

1. The following measured values of loop impedance were recorded for six circuits in an installation. Each circuit is protected by a BS EN 60898 type B cb. Referring to the maximum tabulated values given, show by calculation if these measured valus are acceptable.

2. For any circuit that does not comply, suggest a method of overcoming the situation.

Measured impedance values in ohms:

CT1 CT2 CT3 CT4 CT5 CT6
40A 32A 32A 20A 16A 6A
0.5 1.25 0.96 1.62 2.0 7.3

Maximum tabulated values in ohms (BS7671)
1.15 1.44 1.44 2.3 2.87 7.67


And I am confused!

I have beer in my fridge...maybe I should have one!

Hi Gary.
The zs maximum in bold is before amendment 3 17th edition so out of date.
they added the cmin 0.95 correction factor in 3rd amendment