Why the Limit Here?

hillbilly1

Senior Member
Location
Atlanta,Ga
Occupation
Field coordinator/ technical support
So isn't this essential test gear for you guys? It is for a professional electrician here in the UK. Typical values can range from high hundreds of amps to low thousands in a domestic setting.

We use it in several modes - to determine the PSCC (perspective short circuit current) ie. Will a particular breaker trip on the instantaneous part of the curve if used on this circuit. Also to determine if that breaker has a suitable breaking capacity in order to safely clear such a short circuit fault. A PSCC test can also help identify bad/loose connections.

To determine EFLI (Earth fault loop impedance) - to determine that fault current that would flow if a bolted phase to earth fault should occur. Helps determine the type of earthing (grounding) in use and whether or not additional measures (like an RCD (GFCI) must be used). An EFLI test is done at every point of utilization to determine if the earth connection is present & effective.
Not done here. At most, available fault current at the service, mostly commercial and industrial, never seen it done at the residential level.
 

AdrianWint

Senior Member
Location
Midlands, UK
My understanding is that an RCD is not allowed to "correct" high EFLI in TN-S and TN-C-S systems.

Well, yes & no!

For TN-S & TN-C-S the actual means of earthing is outside the control of the homeowner/electrician. Although, with TNCS the neutral & earth are joined at the service head that join is done by the DNO (POCO), before the meter, and they must not be joined again inside the installation. If a high EFLI is detected it can be very hard to get the DNO to rectify this, the answer is often to "just TT it & fit an RCD". Its very unusual to rely on just over current protection to disconnect in the event of an earth fault.

An RCD is compulsory on a TT system and often used to provide additional protection against shock, indeed in the latest version of our equivalent to the NEC (The IEE Wiring Regulations, 18th edition) the use of an RCD is required on pretty much every domestic circuit. There is much talk of AFCB being introduced here.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Well, yes & no!

For TN-S & TN-C-S the actual means of earthing is outside the control of the homeowner/electrician. Although, with TNCS the neutral & earth are joined at the service head that join is done by the DNO (POCO), before the meter, and they must not be joined again inside the installation. If a high EFLI is detected it can be very hard to get the DNO to rectify this, the answer is often to "just TT it & fit an RCD". Its very unusual to rely on just over current protection to disconnect in the event of an earth fault.

An RCD is compulsory on a TT system and often used to provide additional protection against shock, indeed in the latest version of our equivalent to the NEC (The IEE Wiring Regulations, 18th edition) the use of an RCD is required on pretty much every domestic circuit. There is much talk of AFCB being introduced here.

I get what you are saying, but under not condition should the RCD be the one to achieve 0.4 second disconnection times. RCDs can and do fail.
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Probably because over that distance, there is not enough fault current to trip the breaker in its instantaneous range. But that is just a WAG.

You might be right. Canadian NM:

https://ecat.eleknet.com/PIM_Docs/Docs/STEP_ASSETS_PDF/13560567.pdf


#14 = 3.1 ohms to neutral per 1000 feet at 75*C

#12= 2.0 ohms to neutral per 1000 feet at 75*C

#10= 1.2 ohms to neutral per 1000 feet at 75*C

So

14-14 @ 124.672 feet = 0.77 ohms = 155.8 amps (15 amp breaker)

12-14 @ 196.85 feet = 1 ohms = 120 amps (15 amp breaker)

12-14 @ 164 feet = 0.8364 ohms = 143 amps (20 amp breaker)

10-12 @ 315 feet = 1 ohms = 120 amps (15 amp breaker)

10-12 @ 256 feet= 0.8192 = 146 amps (20 amp breaker)

Given that:


15 x 7 = 105 amps ==== 125% of 105= 131 amps (UL's parellel arc mitigating threshold)


20 x 6 = 120 amps ==== 125% of 120 = 150 amps (UL's parallel arc mitigating threshold)

In reality the above wire will be at 30*C instead of 75*C upon initiation of a short circuit- thus the above current values will actually be markedly higher even when normal voltage varience is taken into account.

It is safe to conclude that these values have a very high probability of tripping a breaker magnetically.
 

romex jockey

Senior Member
Location
Vermont
Older VD threads come to mind, where #10's were calc'd for resi home runs, so here i can see the need for the same rationale for egc's .....~RJ~
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Older VD threads come to mind, where #10's were calc'd for resi home runs, so here i can see the need for the same rationale for egc's .....~RJ~

0.823


Alright, so I asked someone and he/we guessed that at 30*C there would be multiplier of 0.823. At 30*C the AC resistance would be 17.7% lower.

So, without a further ado:

14 AWG = 3.1 ohms per 1000 feet 75*C = 2.5513 ohms at 30*C

12 AWG = 2.0 ohms per 1000 feet 75*C = 1.646 ohms at 30*C

10 AWG = 1.2 ohms per 1000 feet at 75*C = 0.9876 ohms at 30*C
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Older VD threads come to mind, where #10's were calc'd for resi home runs, so here i can see the need for the same rationale for egc's .....~RJ~
Nominal and with -5% voltage at service

GFL = ground fault loop

15 amp breaker:

14-14 = 124.672 feet = 0.6361513472 ohms GFL =/= 188 amps at 120 volts, 179 amps at 114 volts

12-14 = 196.85 feet = 0.826238505 ohms GFL =/= 145 amps at 120 volts, 138 amps at 114 volts

10-12 = 314.961 feet = 0.8294812896 ohms GFL =/= 145 amps at 120 volts, 137 amps at 114 volts

20 amp breaker:

12-14 = 164.042 feet = 0.6885334866 ohms GFL =/= 174 amps at 120 volts, 166 amps at 114 volts

10-12 = 229.659 feet = 0.6048299424 ohms GFL =/= 198 amps at 120 volts, 188 amps at 114 volts
 

mbrooke

Batteries Not Included
Location
United States
Occupation
Electricity
Assuming 2000 amps of fault current Line to Neutral at the panel board we obtain a Ze of about 0.05 ohms...

So adding 0.05 ohms to 0.8294812896 = 131 amps at 115 volts

Adding 0.05 ohms to 0.6885334866 = 156 amps at 115 volts

This is right up to 131 and 150 amps as mentioned in post #64.
 
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