Hello I'm sure someone out there can help me. On breakers that are adjustable thermal/magnetic how do you know what setting to put them on. What do the markings on the breaker mean? (Ir , Im) Can someone please tell me what they mean and how you select the right settings. I am used to using Fixed breakers. Thanks in advance and please excuse my ignorance on the matter.
The method for determining the breaker settings includes performing a protective device coordination study to ensure that devices closest to the overload or fault will act first. The manufacturer will provide you with the time current characteristic curves for the breakers.
I was wondering the same thing RobbieD, I just installed one in a SES the other day. They always seem to come factory pre-set to the lowest setting. I guess that is good enough for me. If it starts nuisence tripping then gradually increase the setting.
But if there is anyone out there who can explain the purpose of these settings in lamens terms. We would appreciate it.
Right guys here we go! Assume that these devices you reffer to are Mccb type! Ir = adjustable overload setting, this is adjustable from 1 breaker full rated current down to .75, eg, 100amp breaker would allow adjustment from 100 amps down to 75amps allowing you to match the breaker directly to the load. As pointed out by Ron these devises ar normally rear end and are only fitted to protect large chunks of an installation or perhaps a large piece of equipment that has its own individual overload and protective devices fitted in the panel. This is where the cascading comes in you need to look at all upstream devices to see at what point they operate then set up your device accordingly to allow them to operate without bringing out the main breaker. Best way to look at it is that the device in the egsample is a 75amp breaker allowing you to trim it up to a 100amps bit like your motor rated fuses.
(Im) Not so sure about we use different terminology (BSEN 60947-2) I think it might be reffering to Short time tripping setting current (isd) this adjusts the instantaneouse tripping threshold. Hope I made sense guys, but I can't stress enough what Ron says though. Do involve the manufacturer they have run destructive tests on all of this gear and produced curves and data to assist with setting up of all of these devices. They are not much use if not set up correctly. Another thing to bear in mind don't use the curve from one manufacturer as a guide to other breakers there can be subtle differences.
I wish it was as simple as telling you to set them to max or min, but it is not that easy. The reason for the adjustably is to work the trip characteristics around the upstream and downstream device's characteristics. There is no easy answer, and generally requires and engineer to perform a study. I will try to find some web links that you can review, but it takes years to be able to get a handle on the art of protective device settings. Many times you have to decide on one extreme regarding reliability and on the other side of equipment protection and reduction of arc flash incident energy. It is not an easy topic because there are many things to consider.
Robbie I fitted a 200A Merlin mccb the other day and wondered the same so asked the question The Ir is the current rating in amps. On my job the wheel was from 0.8-1.This means you can adjust it from 160-200A. Basically you can wind it down to above the design current (Ib) and below the cable rating (Iz)if necessary. The Im adjuster is the magnetic portion i.e instant trip. This equates to the mcb B,C and D type characteristics. My mccb had adjustment from 5X-10X which means it can be changed from B type to a C type.This would be adjusted to cope with large surge loads and the like Therefore this mccb goes in seamless steps from a B160 to C200A in mcb language. I hope this explains it ok!
Thanks everyone! I understood the Ir settings of the breaker but I am still confused about the Im. When a motor is first energized there is a very large inrush current that last for a very short time. This inrush current is very large compared to the FLC of the motor. I think 6x or around there. Am I correct to say that if you are using an adjustable thermal/magnetic breaker for protection of a motor you should set the Im to at least 6X the FLC of the motor?
Most fixed breakers have their instantaneous set to 10xtrip value. Many times, unless there is a coordination issue with an upstream breaker, folks like to set the beaker to it's max. This also results in the highest calculated incident energy for arc flash.
Robbie.. On a motor you wind it up to whatever you like on the Im. As long the max Zs for your adjustment is within the maximum Zs for the circuit ,it dont really matter. There is a question of discrimination of protection but we find at our place that discrimination is only acheived if you use all protective devices by thr same manufacturer