Will a B type 32 trip before a C type 16?

[GBDamo]

Morening all,

I have a dilema that you may be able to help with.

Manufacturers instructions ask for the unit to be fed from a C Type 16Amp MCB. At the moment this is all the technical information I have.

In one in twenty cases they experience nuisance tripping and have now recommended fitting a C type 16 Amp MCB.

Units have previously been fed from 13 Amp FCUs and it would be my intention to feed a locally mounted and enclosed C16 from that FCU.

Ideally I'd like to change the MCB at source but in all likelihood Zs's wont allow that.

Does the brain trust know of a more elegant way to solve this?

Thanks in advance, will look back in later today as on the road at the moment.

 

[Wilko]

Hi - Can you clarify the roles of 32A B, 16A C and the 13A fuse please ? Also, what is the load and what's the existing circuit cable and Zs?

 

[Pete999]

What are the "units"? plus what Wilko said.

 

[Lucien Nunes]

You'll need to compare the curves and understand the type of overload that is causing the nuisance trip. For example, between 18 & 36A (1.13xIn for each MCB) the 16A should trip but the 32A should not, regardless of the type, as only the thermal tripping mechanism will operate. But at 128A (8xIn for B16 and 4xIn for the C32) either or both magnetic trips may act; the tripping time could be anywhere between a few milliseconds and a few seconds and there's no way to predict which will go first.

 

[Risteard]

For example, between 18 & 36A (1.13xIn for each MCB) the 16A should trip but the 32A should not

Below 1.13 In the device must not operate, but at 1.45 In it must operate within the conventional time (1 hour). So between 1.13 and 1.45 In is open.

 

[PEG]

Hi,what is the nature of the load? Inrush issues?

 

[GBDamo]

Hi - Can you clarify the roles of 32A B, 16A C and the 13A fuse please ? Also, what is the load and what's the existing circuit cable and Zs?

Thanks for the reply’s and sorry for the delay in responding.

The “unit” is a dental chair but have not yet seen what or how the load is used or why the manufacturer has now requested a C16 MCB.

Just a panicked call asking whether we can achieve this by putting an MCB in line between the chair and FCU.

I appreciate I need more information but was half hoping it might be a definitive yes or no.

I reserve the right to resurrect this post when I have a clue what I’m talking about.

Cheers though.

 

[Pete999]

Thanks for the reply’s and sorry for the delay in responding.

The “unit” is a dental chair but have not yet seen what or how the load is used or why the manufacturer has now requested a C16 MCB.

Just a panicked call asking whether we can achieve this by putting an MCB in line between the chair and FCU.

I appreciate I need more information but was half hoping it might be a definitive yes or no.

I reserve the right to resurrect this post when I have a clue what I’m talking about.

Cheers though.

I would think the munufacturers requested a C OCPD type because the " unit has inherent inrush problems" I'm going to stick my kneck out here and say, perhaps with your background you should stick to what you know Domestic Installations.

 

[davesparks]

What’s the point in installing a 16A mcb downstream of a 13A fuse?

This sounds suspiciously like the dentists chair is being fed from a ring final circuit.

 

[GBDamo]

What’s the point in installing a 16A mcb downstream of a 13A fuse?

This sounds suspiciously like the dentists chair is being fed from a ring final circuit.

They pretty much all are.

I would think the munufacturers requested a C OCPD type because the " unit has inherent inrush problems" I'm going to stick my kneck out here and say, perhaps with your background you should stick to what you know Domestic Installations.

Domestic installation is one many things I know. I also know that of which I know not enough. Standing still in life bores me I like to learn new things.

 

[PEG]

Is this one of those chairs which have the compressor,vacuum and lighting all contained within?

It sounds like too heavy a draw,for just positional motors.

 

[Marti]

According to 41.3 in the regs, the Zs for the 16amp, type C is 1.37 ohms. 80% of tabulated is 1.1 ohms (rounded). With at least three connections (DB, FCU & local outlet) is 1.1 viable or is the solution a dedicated radial from the DB with the 16 amp type C? Just interested.

 

[NDG Elecs]

I do some work for a dentist that has her own surgery with five chairs in total. As far as I know all chairs are fed from FCUs or plugs. They have three DBs and I can't recall seeing any C curve breakers in any of them. Probably a different brand of chair. The surgery I do work for has a larger central compressor though that feeds all the chairs.