Discrimination Under Earth Fault

[PKJENNY]

Hi,

I have a query regarding an installation where there is an upstream 400A MCCB supplying several 1ph 32A type D MCBs.
According to the manual of the 400A MCCB, the ground fault current setting is set at 80A with 0.2s tripping time.
However, at 80A, the 32A MCB downstream only trip as overload at ~2s, so discrimination fails.

Is the design ground fault current or the tripping time wrong that should be adjusted to achieve discrimination?

Thanks
PK

 

[che12345]

Dear Mr. PKJENNY
Your question appeared in different sections.
Please see below with added PS.
" " che12345 (Electrical)14 Dec 19 04:05
Dear Mr. PKJENNY
Q. "... an upstream 400A MCCB supplying several 1ph 32A type D MCBs. .. the 400A MCCB, the ground fault current setting is set at 80A with 0.2s tripping time. However, at 80A, the 32A MCB downstream only trip as overload at ~2s, so discrimination fails".
A. You may (increase) the 400A MCCB ground fault setting to 0.3In or higher and maintaining at 0.2s, but generally [not] desirable.
You may consider the following alternatives:
a) install [one] 1ph 40A RCD with ground fault rating 30 or 100mA located (up-stream) of the several 1ph 32A MCBs.
Advantage: low cost.
Disadvantage: any ground fault occurred in any [one] of the outgoing MCBs would (blackout) [all] the several MSB outgoings.
PS: Attention, in case that 1ph 40A RCD is not sufficient for the several loads, a higher say 1ph 100A or higher RCD , but rated ground fault within 30 or 100mA may be used.
b) install [one] 1ph 40A RCD with setting 30 or 100mA located up-stream of the [each] 1ph 32A MCBs. Disadvantage: Higher cost.
Advantage: any (one) outgoing MCB ground fault would (blackout) [one],i.e. the faulty branch only. It is Highly recommended/desirable.
Note: 1. the 30 or 100mA RCD shall [not] have any (time delay). This is the usual type available on the market and they are [not costly].
2. the 30mA type would be the preferred rating.
Che Kuan Yau (Singapore) " "

 

[PKJENNY]

Thanks Che Kuan.
It's an original aim not to install any RCD at the downstream 32A MCB circuits to avoid any accidental tripping to the circuits.
I'm just wondering why the MCCB ground fault setting is recommended to set as low as 0.2In? I think the ground fault current at the 400A MCCB location won't be such low, but should be at least a few hundred amps?
Also, alternatively, the 0.2s tripping time would be too low? I think a setting of below 5s would still comply.
I am thinking either setting at a higher %In or higher tripping time would be more appropriate?
Am I right?

Thanks
PK

 

[RRaghunath]

The earth fault current can be a few amperes to many kilo amperes, depending on the source connected to the switchboard.
When there is no dedicated earth fault protection in the outgoing 32A feeders, it is not possible to ensure coordination with the earth fault protection available with 400A MCCB.
If an RCD or any other fast acting earth fault protection is not acceptable in 32A MCB feeders, we need to accept the compromise that there is a risk of 400A MCCB earth fault protection isolating the MCCB for low magnitude earth fault currents.
Obviously, as long as the earth loop impedance is low the earth fault current magnitude is likely to be large, ensuring MCB trips immediately.

 

[waross]

In some jurisdictions the main breaker is a GFCI breaker and the downstream breakers may be non-GFCI breakers.
It is an economical way to provide ground fault protection to an entire building.
I have seen utility supplied and sealed GFCI main breakers with a 0.030 Amp ground fault setting.
If a consumer wanted to add co-ordination he could pay the price for 0.015 Amp GFCI branch breakers.
I agree with Mr Che Kuan Yau on his suggestion of 30 mA protection. (0.030 Amp).
That provides good equipment protection and some measure of personal protection.
Downstream breakers at 15 mA will provide both co-ordination and life safety protection.
I don't see much advantage in an 80 Amp trip in 0.2 seconds.
That will allow a lot of damage before clearing.
A 40 Amp fault will do a lot of damage before the 32 Amp branch breaker trips.
I suspect that your main breaker is intended for use on three wire circuits and does not meter neutral current.
The 80 Amp setting will allow single phase to neutral branch circuits to operate without tripping as long as the total neutral current does not exceed 80 Amps.
A GFCI breaker intended for line to neutral loads will have a fourth, pass through, terminal for the neutral.
The neutral will net be switched but will be included in the current loop for the GFCI function.
An 80 Amp setting of a GFCI breaker seems pointless.
Do you have a feeder cable with a very high charging current?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. PKJENNY

P1. "It's an original aim not to install any RCD at the downstream 32A MCB circuits to avoid any accidental tripping to the circuits".
C1.1 As a responsible engineer,it is our duty to (educate) the consumer of the advantage of life/fire safety offered by RCD. Consider the alternative b) suggested in my earlier post. It would [shut down only the faulty branch], [not] all the several branches; which the consumer (may have had [bad ]experience with previously). The additional cost is (not) exorbitant. It is a form of buying an insurance. Opinion: highly recommended/desirable.
C1.2 You may set the 400A MCCB ground fault to 0.9In or OFF. Such that it will [not] operate on any level of ground fault, if that is what the (ill-informed) consumer wants.
Caution: Attention, this irresponsible approach which may satisfies the consumer but it is HIGHLY UNDESIRABLE and or DANGEROUS.

P2. " I'm just wondering why the MCCB ground fault setting is recommended to set as low as 0.2In? I think the ground fault current at the 400A MCCB location won't be such low, but should be at least a few hundred amps?"
C2. You can also set the 400A MCCB ground fault to 0.9In or OFF. It will [not] operate on any level of ground fault.

P3." Also, alternatively, the 0.2s tripping time would be too low? I think a setting of below 5s would still comply ".
C3. You can set the 400A MCCB ground fault to OFF. It will [not] trip on ground fault at any level. It would however, (coordinate) only on [over-current] but not on short-circuit or ground fault.

P4. " I am thinking either setting at a higher %In or higher tripping time would be more appropriate? "
C4. Setting to higher %In or higher tripping time is unlikely to ensure a full ground fault coordination as the ground fault level varies from very low to very high level. Opinion: NOT desirable.
Che Kuan Yau (Singapore)

 

[PKJENNY]

Noted and thanks waross and che12345.

I just wonder if a setting of higher than 0.2s is possible, given a fixed installation 5s disconnection time would comply with IEE?

 

[che12345]

Dear Mr. PKJENNY

Q. "I just wonder if a setting of higher than 0.2s is possible, given a fixed installation 5s disconnection time would comply with IEE? "
A. I am of the opinion of retaining MCCB GF at 0.2s and add 40A (GF 30mA) RCD on every MCB outgoing would be the better solution. In this way the system from MCCB outgoing up every MCB loads are [individually] GF protected.
Proposal: " educate " the consumer of the importance of RCD. The additional cost would not be exorbitant. Any one MCB outgoing GF should switch off (only the faulty branch),[which is a necessity anyway]. It is (dangerous) and [shall not] allow GF >30mA to persist for any time delay; if it is for life saving.
Che Kuan Yau (Singapore)

 

[ScottyUK]

PKJENNY,

You could consider an external earth fault relay to trigger a shunt trip within the MCCB if you cant get the range of adjustment necessary on the MCCB itself, although most have interchangeable tripping modules of varying levels of complexity - Schneider, Siemens and ABB all have a fair few tripping modules to choose from. Something like a MiCOM P120 single phase overcurrent relay, for example - it's not a particularly expensive relay.