MCB Query

Lex Luthor

I currently have a requirement to install a few heaters into an enclosure that will be controlled by thermostats.

Due to the specific characteristics of the heater the element has a switch on current of 6A for approx 1 sec and then has an operating current of 0.5A.

The manual states I need to have a pre-surge fuse of 4A for this to operate.

I am told I need to get a 'C' rated MCB for this application. So would this mean I need a 4A 'C' rated mcb?

Any sparks out there help me out on this?

Mr Magnolia

B = above 3In up to and including 5In
C = above 5In up to and including 10In
D = above 10In up to and including 20In

Where In = instantaneous tripping current.

So, if all this is correct you could get a 'C' type, 1amp mcb. This would cover you for an initial current of 5-10Amps. The suppliers will have a chart on the type of mcb's they offer. Though I doubt it, values may differ.

Lex Luthor

Mr Magnolia wrote: »

B = above 3In up to and including 5In
C = above 5In up to and including 10In
D = above 10In up to and including 20In

Where In = instantaneous tripping current.

So, if all this is correct you could get a 'C' type, 1amp mcb. This would cover you for an initial current of 5-10Amps. The suppliers will have a chart on the type of mcb's they offer. Though I doubt it, values may differ.

The 1A C rated might just be a little too low as there is a surge of 6A on start up. Maybe a 2A C type could do then?

10-10-20

What is the surge cause by? Motor, transformer or what? It's hardly the heating coil? Would a suitable mains choke (snubber) be of any use?

fishdog

How about feeding each heater from a spur outlet with a 3A fuse in the spur outlet? The circuit with the spur outlets could be protected by a 20A B type MCB. I dont know how many heaters you have, but maybe you can have all of the heaters on the same 20A circuit.

Why is there 6 amps initially?? Is this just because the element is cold and has low resistance??

Sounds a bit strange, but I think the above solution will do it. If the fuse blows, try a higher fuse.

Mr Magnolia

Lex Luthor wrote: »

The 1A C rated might just be a little too low as there is a surge of 6A on start up. Maybe a 2A C type could do then?

The 1amp 'C' type will cover initial surges between 5-10amps. Your surge is 6amps implying the 'C' type is best suited, unless I'm missing something. Don't think I am though.

fishdog

The 1amp 'C' type will cover initial surges between 5-10amps. Your surge is 6amps implying the 'C' type is best suited, unless I'm missing something. Don't think I am though.

Install an MCB for each heater?? I dont know how many heaters there are, but what you are saying is one circuit per heater that only draws .5 amp when it is running. Why??

You could easily have sevral heaters on the same circuit (perhaps 20A B type MCB wired in 2.5) each protected by its own spur outlet. This would save alot of space in the board and make it more economical.

Lex Luthor

There is currently 1 heater installed on a 2A MCB that has a running current of 0.5A

I need to put a 2nd heater on this same cicuit as the heaters are very remote to the panel that is supplying them. The new heater will have a running current of 1A but will be switching in & out due to the stat that will be controlling it.

I don't have the option of putting each heater on a separate MCB as the customer isn't prepared to pay to get another cable installed. The new heaters data sheet just mentions that it has this 6A switch on current for 1 second. I'm not sure of the design as to why it does this. Its a fan style heater.

So in order to prevent the original 2A MCB from tripping each time the new fan kicks in, I just need to find out what MCB I should be replacing it with to cope with the extra load.

fishdog

The first thing you have to do is look at the current carrying capacity of the cable. What size is it, how long is the run and what is the total load on that cable? These will be the most important factors. You should check your stat also, is it rated to switch 6 amps??

Perhaps there is a local socket circuit you can feed the heater from, it is a small load.

Why dont you do a tempoary connection to the 2A MCB just to see if it can take it??

BTW a 2A MCB sounds a little odd!

Lex Luthor

fishdog wrote: »

The first thing you have to do is look at the current carrying capacity of the cable. What size is it, how long is the run and what is the total load on that cable? These will be the most important factors. You should check your stat also, is it rated to switch 6 amps??

Perhaps there is a local socket circuit you can feed the heater from, it is a small load.

Why dont you do a tempoary connection to the 2A MCB just to see if it can take it??

BTW a 2A MCB sounds a little odd!

I'm not going back to site to fit this...the contractors on site will be doing the work over the Xmas shutdown. They just want me to supply them the breakers for the heaters that ze germans supplied them. Its all too confusing.
The cable size is sufficient for the total load so I am told by ze germans.

The stat is rated to take the 6A.

Why do you say the 2A MCB is odd?

fishdog

Why do you say the 2A MCB is odd?

There is nothing wrong with a 2A MCB, it is just not one that you would come across that much.

So (if I am eading this correctly) one cable for 2 heaters and the cable can take the load for both?

Then if a 2A MCB can supply 1 heater a 4A could supply 2. I dont think you will be able to get a 4A MCB, so put a 6A on.

If the operating current is only 0.5 amps it will not give out alot of heat (about 115 watts). What kind of heater is it??

Lex Luthor

Current heater is about 200W on the 2A MCB. Its a heat trace cable about 2m long and is always on.

The new heater is a 130W Rittal 3107 enclosure heater.

So when you say the 6A MCB should do, will that be able to handle the 6A surge on startup of the Rittal heater plus the 0.9A of the heat trace?

0.9A (Original heater on all the time) + 6A startup = 6.9A which is greater than 6A.