Merlin Gerin MCCB Zs Values

Hi All,
We have a Merlin Gerin Compact NS100N Single Phase MCCB
I've been asked to find its Max ZS Values and to be honest I cant find anything!

This is an from an old MCCB board we are re testing. I put it in google and cant seem to find anything, I know Schneider bought this brand out. But their website doesnt have anything from Merlin On, that I can find.
Any assitance with finding this out? Or Can someone help me Calculate the Max ZS?

Or would I use the following Calculation

230 x 0.95 = 218.5
218.5 / 500A (lm - Short circuit Instantanious) = 0.437

0.43 x 0.8 = 0.349 (80% Value since Cold Circuit?)


@Julie. This is the calculation you showed me the other day, would this be applicable?

If it is a final circuit then yes, you would have to hit the "instant" magnetic trip point and that appears to be 500A fixed. however, what you computed is the resistance for a working (i.e. "hot") circuit so you either have to measure it just off-load so the cable is at it typical operating temperature, or you scale by 0.8 to allow for it measured cold on inspection but hot when a fault might occur. So yes worst-case meeting 0.35 ohms would do.

If it is feeding a sub-main to another DB then you would be looking at 5s max so a slightly higher Zs but some more info on the thermal curve would be needed to determine just what is acceptable.

Above times assuming a nominal 230V L-N TN system.

pc1966 said:

If it is a final circuit then yes, you would have to hit the "instant" magnetic trip point and that appears to be 500A fixed. however, what you computed is the resistance for a working (i.e. "hot") circuit so you either have to measure it just off-load so the cable is at it typical operating temperature, or you scale by 0.8 to allow for it measured cold on inspection but hot when a fault might occur. So yes worst-case meeting 0.35 ohms would do.

If it is feeding a sub-main to another DB then you would be looking at 5s max so a slightly higher Zs but some more info on the thermal curve would be needed to determine just what is acceptable.

Above times assuming a nominal 230V L-N TN system.

Click to expand...

Okay, so this is feeding another Cabin DB.

Can i ask;

1) Why does it need to be 5s, I was under the impression it should be 0.4 second disconnection times. (Sorry Still learning)
2) What's my calculation if it is 5s rather than 0.4?

QuestionableSpark said:

Okay, so this is feeding another Cabin DB.

Can i ask;

1) Why does it need to be 5s, I was under the impression it should be 0.4 second disconnection times. (Sorry Still learning)
2) What's my calculation if it is 5s rather than 0.4?

Click to expand...

Because it's not the final circuit.

If you are feeding another distribution board, then the feeder needs a disconnection time of 5s

Final circuits it's 0.4s

(Of course there are other times for tt and so on)

TBH on a feeder ideally set it so there is no high set (just the curve section), or introduce a delay on the "instantaneous" - high set part to meet the 5s

I can't see from the photo, but often there is a section allowing you to select values - we would need to see them to see if your calculations are appropriate.

You also need to know the trip unit fitted, it could be the thermal unit, or an electronic version, but from that you can get the curve similar to this:

Julie. said:

Because it's not the final circuit.

If you are feeding another distribution board, then the feeder needs a disconnection time of 5s

Final circuits it's 0.4s

(Of course there are other times for tt and so on)

TBH on a feeder ideally set it so there is no high set (just the curve section), or introduce a delay on the "instantaneous" - high set part to meet the 5s

I can't see from the photo, but often there is a section allowing you to select values - we would need to see them to see if your calculations are appropriate.

Click to expand...

Thankyou again Julie! I have seen the other times for a TT so i am aware for the different earthing arrangements.

Everything on that pic is in view, I couldnt see anywhere to change any settings. - On 3phase MCCBs ive come across it is usually the case that I can change these settings with a little dial.

QuestionableSpark said:

Thankyou again Julie! I have seen the other times for a TT so i am aware for the different earthing arrangements.

Everything on that pic is in view, I couldnt see anywhere to change any settings. - On 3phase MCCBs ive come across it is usually the case that I can change these settings with a little dial.

Click to expand...

MCCBs can be fitted with several trip units, you need to know which one to see exactly what curve is applicable.

Most have the dial, but even if fixed, you still need the curve

QuestionableSpark said:

1) Why does it need to be 5s, I was under the impression it should be 0.4 second disconnection times. (Sorry Still learning)

Click to expand...

Julie partially answered this, and really is doesn't need to be 5s rather it can be up to 5s and still compliant. There are two aspects to this:

• The longer time is often part and parcel of OCPD selectivity: you cant have up and down stream devices disconnecting in the same sort of time-scales on a big fault and still have the ability to isolate a faulty final circuit. Sometimes such delay is fixed and deterministic - like RCD where the 'selective' type has a deliberate ~0.2s delay on any large fault - or it can be a much higher OCPD so the response time for a given PFC is longer.
• The allowance for a longer time (and so greater period of personnel at-risk of shock) is deemed acceptable as sub-mains are usually fixed and properly routed so the probability of a fault coinciding with someone operating a given class I appliance is acceptably small.

The other times for TT, etc, come down to the risk of fatal shock. In a TN system R1 & R2 are normally of similar magnitude, so during a L-E fault anyone in contact with the appliance and true Earth sees about half the supply voltage, whereas TT has a very much higher R2 (due to the rod's Ra being tens of ohms or more usually) so such faults cause almost all of the supply voltage to appear on the CPC for the time it takes to clear the fault.

QuestionableSpark said:

2) What's my calculation if it is 5s rather than 0.4?

Click to expand...

Difficult for MCCB.

For MCB you normally only see 5s being applicable for D-curve devices and it is roughly double the Zs for 0.4s, for MCCB they are often similar, but without the manufacturer's curves you can't really determine the PFC limit for <= 5s disconnection.