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rajsiyer

Heater & SSR protection

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Hi Friends, These days, I am in the midst of designing a control panel. The panel has around 20 of heater loads, all 3 phase 480V with capacities from 7-45kW. I plan to use SSRs for each heater. To protect the SSRs & the heaters, I plan to use OMRON's K8AC-HXXX burnout detector. The panel also has 5 small motors all less than 5HP. This control panel has to comply with the requirements of UL508A. Coming to my design...In the circuit of each heater, there will be a MCCB (rated at 125% of heater's FLA), for Short-Ckt protection of the feeder. Downstream to this CB, is a contactor interlocked with the burnout detector. After the contactor, comes the 3-phase SSR. The outputs of these are taken to the terminals for heater connections. There are CTs in 2 phases, between SSRs & terminals to sense the load current for the Burn-out detector. The burn-out detector is powered up from the control voltage (120VAC) . This device is intended to detect the SSR's Open-circuit /Short-Ckt failures as well as the heater's opn-ckt & abnormal currents. Should any of these get detected, the contactor will open. My questions are ... 1. Can a single common control-power transformer be used for all contactors and the power-requirements of the burn-out detectors?. This question is because the OMRON K8AC** provides compensation for fluctuations in input voltage, which is as seen at its own power-supply terminals> 2. Must the panel be of a compartmentalized type? 3. Must the control power transformer (approx rating 1kW) secondary be earthed?? 4. What is the correct way to protect the control power transformer. 5. How good/reliable is the OMRON device?? Are there any other makes for similar burn-out detectors?? Regards, thanks a million,

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Here are some answers/views on some of your questions. 1) One control transformer should be okay. You may need to increase the size of transformer for the inrush current of the contactor coils since you have so many of them. Some contactors have inrush currents 10x greater than thier hold in currents. If several contactors energize at once you could see very significant voltage drops if the transformer is undersized. 2.)Since you have a UL508A requirement you must use a UL listed enclosure for the environment you are going into. Dry /non-dust = NEMA1, Dry/dusty = NEMA 12, Wet = NEMA 4, Wet/caustic or acitic = NEMA 4x, outdoor rainproof = NEMA 3R. 3.) UL508A & common practice in the US dictate that one side of the transformer secondary be tied to ground. That will be the white neutral phase. There are exceptions if there is a real necessity to have an ungrounded secondary (then both phases would be red). Best to ground if possible. With ratings over NEMA one you need to pay attention what holes you punch into the cabinet. If there is a hole it has bto be filled with a UL device rated for the environment. You cannot just punch a hole in an encolusre to mount it. 4.)To protect a 1KVA transformer you will need Short Circuit protection on both primay phases and 1 fuse/CB on the non-grounded secondary. The secondary is sized @ 125% (max). The primary is 175% max (fuse) or 250% max (thermal CB). Primary fuses/CBs should have slow trip caracteristics to avoid blowing the priimaries when power is shut off (kick-back current). 5.) never used the omron device. Years ago protecting SCRs from short circuit was difficult. The SCR would blow before the high speed fuse or CB. Some new SCRs are self protected from downstream faults.

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Thanks bimini3, Your advice exactly addresses my concerns. Yours + those of the earlier gent have made me deiced confidently to eliminate the Omron device. I shall be using Short-Circuit protection on each heater using a breaker rated 1.25 times its load current + one OLR having a range with load current towards the higher end of its scale. Since there are 23 contactors in all, 22 having about 30VA + one contactor with 158VA + 3 indicating lamps on each branch, I've chosen a 1kVA control transformer, with a DP 5A rated ckt. breaker on the primary and 15A fuse on the secondary. Each contactor's control ckt. will also have a 1A fuse. Thanks for the NEMA explanations. I'll choose accordingly. Thanks and best wishes for your happiness. Raj S. Iyer

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