Our Odyssey

Automatic Transfer Switch

Since you're on this page, I can only conclude that you are either a glutton for punishment, or a complete electrical geek. (Or, maybe, you're doing a bus conversion yourself.) In any case, I suggest you settle in for a long discourse. First, open up the diagram, which will open in a new page. You will need to refer to this throughout the discussion.

Before getting into the actual description of the ATS, you should, if you have not already done so, read the discussion of the selection of the SW4024 and the issues around using the standard (vs. the MC2 RV-specific) model, here. Reiterating some of the information from the AC Power System page, the design goals of the ATS are:

  1. Accommodate minimum 70-amp service, to accommodate the generator's 70-amp output and the SW4024's 60-amp input.
  2. Make the generator the preferred AC source whenever it is running, even if shore power is available.
  3. Permit seamless use of the SW4024 generator autostart features. This requires that the SW4024 "see" the power from the generator even as it is starting.
  4. Allow the SW4024 to start the generator as the batteries drain, even if it is already connected to shore power. (The batteries could very well be draining if, for example, the connected shore power is only a 15-amp service.)
  5. Connect the Shore input to AC1 and the generator to AC2 on the SW4024, allowing different "sizes" (ampacity) to be set for each.
  6. Ensure that one, and only one, ground-to-neutral bond exists within the coach when running on generator power or inverter-only power, but none exists when running on shore power.
  7. Provide a start-delay for the generator for the "dry side" loads (inverter load is delayed by a settable timer in the SW4024).
  8. Detect the presence of 240-volt service and connect "dry side" loads to the shore cord only when same is present.
  9. Alert the "Energy Management System" when shore is connected and whether it is 120-volt-only or 120-240.
  10. Comply with all NEC provisions and operate safely at all times.

That's a long list, and, it turns out, a fairly tall order. I went through quite a number of design iterations before settling on the one in the diagram. I won't bore you with all the details, as they are not relevant, but a couple of items are worth mentioning. 1. I started out, after noodling through the list of requirements, with a design that involved a 4PDT relay. This would have been ideal, except for the small detail that it would have to carry the full 60 amp load of the SW4024. Perhaps you will have more luck, but after many hours of searching, I determined that such a device simply does not exist. I mention this here to save you the trouble, if you happen to be studying up for your own project. 2. As we get into the diagram, you will note that the generator side of the transfer contactor is interrupting only one hot leg, and the other hot leg as well as the neutral "pass around." Originally, the generator contactor switched all three, and the generator had it's own ground-neutral bond. The problem with this arrangement was that no time delay would be permitted in closing the generator contactor, as the SW4024 needs to "see" power come through from the generator to know when to stop cranking in autostart mode. We wanted to use the autostart, but also to implement a transfer delay for the part of the load not connected to the SW4024, so I elected to bypass the contactor for the feed to the SW4024 only. That necessitated removing the ground-neutral bond from the genset, and letting the ATS do the bond when running on generator power. One nice thing about having designed it this way, and used three-pole contactors in the implementation, is that we can always go back to passing all conductors through the contactor if we need to (such as replacing the genset with one where the bond can not be removed), requiring only that we remove the time delay from the transfer.

Moving on to the actual implementation as it is currently installed, the components are:

The reversing motor starter is the centerpiece of the transfer switch. On the diagram, this is the pair of dashed rectangles connected by a diamond symbol. For those not familiar with reversing motor starters, this is simply a pair of three-pole high-current contactors (in this case, rated for 80 amps at 600vac), which are mechanically interlocked so that, irrespective of any voltage applied to the coils, only one contactor can be "closed" at a time (the other contactor is physically prevented from closing by the mechanical interlock). The contactors are also electrically interlocked, making the physical interlock a redundant safety mechanism. The electrical interlock consists of smaller "auxiliary" contacts on each contactor. The operating current for the coil of one contactor must first pass through a "normally closed" (N/C) contact on the opposite contactor. This ensures that the contactor will only try to "close" when the main contacts on the opposite contactor are "open." To keep the drawing legible, this electrical interlock is not shown -- the drawing instead shows the operating signal going directly to terminal "A" of the respective contactor. (That's why the "A" terminal is marked "See Notes.") Also not shown are the neutral returns for the operating coils, which are simply connected to the neutral bus, and grounds (other than at the bonding relay), which are permanently connected for all inputs and outputs.

The upper contactor is for the generator feed, and, as discussed above, is currently only "switching" one of the two hot leads, while the second hot and the neutral pass around it. This means that the generator's neutral is connected to the neutral bus at all times, even when the generator is not running. This required the neutral-ground bond to be removed from the generator, otherwise a duplicate bond would exist when connected to shore power (where neutral and ground are again bonded). It is beyond the scope of this page to go into the details of why neutral and ground can only be bonded in one place -- suffice it to say that it's dangerous otherwise, plus any GFCI protection on the shore circuit would trip. One implication of this is that the generator neutral must also be continuous and hard-wired all the way to the bonding relay in this transfer switch (otherwise the generator could be dangerously ungrounded). This is a common configuration in fixed standby generator installations, and is safe (and code-legal). As discussed above, the reason for bypassing the contactor with the hot lead feeding the SW4024 (thus necessitating the pass-through of neutral as well) is so that the SW4024 can "see" power coming from the genset on start-up, so it will know when to stop cranking. The SW4024 also begins counting down its transfer time-delay once it sees stable power on this input.

The next contactor down is the Shore Power contactor, interlocked as described to the Generator contactor above. (But, I hear you ask: What good is the interlock if some of the power is routed around the Generator contactor anyway? Answer: The hot lead that bypasses the contactor feeds only the SW4024, and it does so on input "AC2" thereof. Similarly, the hot lead coming from the Shore side passes to the "AC1" input of the SW4024, which has its own "transfer switch" internally to select between these. In other words, even though both inputs may be hot at once, the SW4024 will only select one of them, and they will never be cross-connected.) You will note that, for the shore line, all three current-carrying conductors pass through the contactor. Indeed, the need to interrupt not only the two hots but also the neutral is what drove the selection of "3-phase" contactors in the first place (note that, while designed for 3-phase applications, these contactors are rated and listed for single-phase applications as well). This is an important feature -- it allows the shore cord to be physically connected to the coach without the shore cord's neutral being electrically connected. The neutral is electrically connected to the neutral bus only when the shore input is selected by the ATS and shore power is being fed through.

We will skip the third large contactor, shown below these two, for now, and instead describe the three smaller relays and how all this operates together. First, note that there is a small line shown connected to the input side of one of the (switched) hot legs on each contactor, and from there feeding a small 5-amp circuit breaker. These lines are the initial derivation of the "control signals" that make the ATS work. Starting with the control signal from the shore contactor (light blue), we see that the signal divides into two paths after the breaker. One path takes the signal through a pair of normally open (N/O) auxiliary contacts on the shore contactor itself (each of these contactors comes equipped with one N/C and one N/O contact pair -- the N/C pair, as described above, is used for electrical interlock, and the N/O pair is available for other uses. These contacts are rated at 10-amps each.). After passing through the N/O contacts, this signal operates the coil of the neutral-ground bond relay shown just to the right of the neutral bus, thus "breaking" the internal neutral-ground bond. This signal also passes to the "A1" coil input of the third high-current contactor, which will be described later. The neutral-ground relay is actually a P&B 30-amp DPDT relay with the two poles paralleled.

The other path for this control signal takes it through the N/C contacts of the 24vdc relay and the generator delay timer before landing (by way of the electrical interlock described above) on the operating coil terminal of the shore contactor. The normal sequence, when shore power is connected, is as follows: Control power passes through the N/C terminals of the 24vdc relay, the generator timer, and the idle generator contactor and flows to the operating coil of the shore contactor, causing the contactor to close. Once closed, control power then passes through the N/O aux contacts of the shore contactor to the neutral-ground bond relay, thus breaking the bond. Note that this means that if the shore cord is connected, but, for whatever reason, not energized (e.g. due to a tripped breaker in a campground power system), this contactor will drop, thus breaking the electrical connection to the shore source's neutral, and the internal ground-neutral bond will be restored. This prevents double-bonding when shore power drops out for whatever reason.

If either the 24vdc relay or the generator timer should open, the coil voltage to the shore contactor will be immediately interrupted and shore power will be disconnected. This allows the generator, when running, to assert priority. It also allows the SW4024, by means of the 24vdc relay, to drop shore power when the batteries drain below a set point. This is required because, with limited shore power available, the demands on the batteries may cause them to discharge faster than the shore power can keep up (for example, 20-amp shore power connected, but two air conditioners running for a total of 25+ amps being used in the coach), however, the SW4024 will not start the generator because a valid 120-volt source is connected to AC1.

Moving to the control signal for the generator side (red), we see that the signal passes immediately to the operating coil of the Generator Delay Timer, as well as to one of its N/O contacts. This timer will energize after a fixed (user-selectable) time delay, thus passing the control signal through to the operating coil of the generator contactor (by way of the electrical interlock). Simultaneously, the generator delay timer (which is actually a DPDT device) interrupts the operating current, if any, for the shore contactor. This ensures that, after a brief start-up delay, the generator will be connected to the loads irrespective of the availability of shore power. This also forces the SW4024 to use generator power whenever the generator is running.

This brings us to the large contactor on the bottom-left of the drawing. This is another 80-amp 3-pole contactor, physically identical to the two above it, except, as a stand-alone contactor, it lacks the mechanical interlocks. It also has a 240vac operating coil, as opposed to the 120vac coils of the transfer contactors. The coil operating inputs, A1 and A2, are connected to the two hot legs of shore power when (and only when) the shore contactor is energized. If the shore cord is actually delivering 240-volt power, then this contactor will energize, otherwise, it will remain open. The effect of this is that, when on shore power, the "dry" (non-inverter) load center, "Panel N," will only be energized when a "full" shore feed is available. (Generally, 240 volts present means a 50-amp shore service connection, while 30-amp/120-volt and smaller shore services will have both "hot" legs tied together, with zero voltage between them.) Since we can completely use up anything smaller than full 50-amp/240-volt service with the SW4024 and its loads alone, this is an important part of our load management system.

The other part of our load management is our Energy Management System (EMS), located in our load distribution center. The N/C auxiliary contact of this 240-volt contactor passes the "shore present" signal on to the EMS only when the contactor is unenergized. The meaning of this signal (labeled "To Load Control Relay" on the drawing) once it leaves the ATS, is "shore power is connected, but it is only 120-volt and therefore half the size (or less) of a full service". See the separate description of the EMS for how this signal is used.

A final note about this last contactor is that it has, like the others, three main contact pairs, but we are only using one. This is simply due to the design efficiency of using parts of a like design and footprint. Since the parts of these contactors are all interchangeable, pieces can be swapped between them in an emergency, and sourcing spares is simplified. There is no plan to use the two unused contact pairs (unlike those on the generator contactor, which may be needed if a different generator is installed).

The only remaining note on this diagram is the signal labeled "To Genny Fan Control System." This is actually obsolete, as the fan system has been redesigned.

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