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DLB

Circuit Protection

6 posts in this topic

Hi, I am setting up a small PLC test rig in the office workshop with an AB ControlLogix Safety PLC but am a bit confused over some information I received re circuit protection. I have been told that I don't need an RCD because the buildings ring main is protected by RCD's. I was also told that I don't need an MCB to protect the PLC as the RCD is in place on the ring main. I questioned the fact that the RCD protects against earth leakage but not short circuits/overcurrent and was asked "Well what short circuit are you wanting to protect against? The RCB will take care of it". ....this is wrong, right? I didn't question it as I wanted to get my facts straight before I did. I thought I needed an RCD and MCB, to which I looked for an RCBO so I just had one unit. The PSU (1756-PA72) draws 0.416amps (using I = W/V = 100/240 = 0.416A). So, I need a type C MCB sufficient for that amount of current. Is this correct? Also, if it turns out that I do need an MCB I was unsure how to account for inrush current? The PSU is rated at 20A max inrush. I found this DIN rail mountable RCBO but was unsure if the 10A trip rating was too high? http://www.screwfix.com/prods/33098/Electrical-Supplies/Consumer-Units/Wylex-MCBs-Incomers/WYLEX-10A-30mA-SP-Type-C-Curve-RCBO# Any help would be really appreciated :) Edited by DLB

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All circuits should (must) have overcurrent protection. Components should have it but there comes a point at which not cost effective ie you dont fuse individual lite bulbs. If you are in a warm and dry office settiing I see no reason for ground fault protection. Over current protection will protect against ground fault BUT at current levels much higher that ground fault protectors (GFCI) will. GFI are designed to protect people not the circuit. I suppose you can get units that will do both. Dan Bentler Edited by Leitmotif

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Partly true, partly not. First it depends on the type of grounding. If you have ungrounded deltas, you better have ground fault detection as a minimum but unfortunately (and this is one of the big problems with ungrounded deltas), you can only really detect that there's a ground fault on one phase SOMEWHERE in the system. So you have to manually start opening breakers and switches to find it. With resistance grounding, you can detect and control ground fault, and even use timing in the protection relays to do coordination (isolate it). With "solidly" grounded systems, this can still be done with core-balanced CT's (hence the GFI's to protect people) but normally, one relies on the overcurrent protection to do double duty. The problem with overcurrent protection is this. Suppose we have a 230 A long term setting circuit breaker, which is not unusual for a fairly large motor. What ground fault current is required to trip it? Suppose we have a 460 VAC system. If we have a dead short to ground, what is the maximum impedance of the bonding back to the transformer which will trip the breaker? 460 / 230 = 2 ohms. Now try checking your actual system to see what your typical resistance actually is....that's where the problem is. And it only gets worse as the voltage and current go up. And typically you can expect the current draw to only be about 75% of FLA during an arcing event even on 480 VAC systems, forcing the ground resistance even lower. So...at the level of a PLC, ground fault is easily done. In large industrial systems though overcurrent protection by itself doesn't usually work in reality. Hence the reason that in medium voltage systems, it is all but required by code as well as in practical reality.

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Not sure what terminology you are seeing as cryptic? Maybe US terminology is completely different to that of the UK? This might be the case as some of the terminology you used was pretty cryptic to me. I think both replies unfortunately went into way too much detail for what I was asking. Maybe this is my fault for going into too much detail on my post. I've found a little more info now. The ring main it will be connected to has an MCB at the consumer unit, but no RCD. I have simply purchased a plug in RCD with a tripping current at 30mA leakage. The breaker in the consumer unit is a type C and appears to cope fine with the inrush and there are no other critical pieces of equipment on the ring main that would be tripped if the PLC was to trip the MCB.

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Got it. UK=RCD. US=GFI. UK RCCB, US=GFCI. In the U.S. we have to trip "people protection" devices at 5 mA or less. In the UK, this can be up to 30 mA. In the UK, it is required pretty much everywhere. In the U.S., they are location specific (don't need it everywhere). But in addition, in the U.S., they are required for equipment in certain mining applications as well as high power distribution systems (think 1000 A or bigger), but in this case, it's to protect equipment, not people, and the trip setting is much larger. In addition with three phase power distribution, there are several options as to how the system is configured. Certain ones (ungrounded delta) are not even capable of residual current to ground in the event that a phase leg touches ground. Others (solidly grounded wye) have a maximum amount of current flowing to ground in this event, or anything in between (resistance grounding). If you have anything other than the solidly grounded type (and even then, not always), RCD's are needed in this case to isolate the fault.

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Makes more sense now :) Thanks Paul!

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