Alex,
A surge protection device has the ability to protect a circuit against a fast rising voltage, (induced current), from numerous sources. The devices, (IDEALLY), need to be customised to provide the maximum protection to the intended circuit, as an example, some commercially provided SPD devices are triggered at 1000V. If your circuit is sensitive to voltages over 150V, then this device is not going to offer much protection. It will help when a voltage surges above 1000 volt but will remain inactivated for a surge of 800V.
I would recommend that you consider the highest voltage that will be expected on the line, (I didnât see a voltage on your circuit), if it was a 48V nominal battery connected to the Lynxs distributor, then I would be looking for an SPD that would activate at no more than 75V.
With regard to the protection offered and the location, you have to consider WHAT you are protecting against and WHAT is to be protected. In your circuit you have an AC/DC charger, (this looks like the main source of power by the amperage rating of the fuse). If you are concerned about spikes on this source then the SPD should be located on the AC side of the charger, preferably as close to the connection point of the AC cable. This would allow all the cable from the connection point to assist in the reduction of the surge on the AC line, think of the cable as a resistor/inductor will help reduce the surge.
If the surge was from the DC/DC chargers, ie an inductive surge if the battery was disconnected, (as per the question I responded to), then the SPD should be located as close to the DC/DC output as possible.
If the surge is from the MPPT unit, (assuming solar panels), then the SPD should be located on the solar panel side and be about half way between the panels and the MPPT, (or close to the MPPT to keep it out of the weather). The reason for the half way point is because a nearby lightning strike will induce a current and hence voltage into the cable, it works like an aerial, the mid point will absorb spikes better, protecting both ends equally.
If the surge is, (like the article I responded to), is caused by the battery being removed and inductive spikes resulting, then you would place near to any inductive sources, (as per DC/DC charger above).
The important part to remember is that the SPD should be fabricated with consideration of your voltage limits. If installed in a solar string, then the activation voltage should be set close to Voc +10%. If installed on a battery line, then Vbatt(max), +10%. For DC applications like the battery, you would only need a varistor connected from the positive to the negative line, for an AC line or the solar panels, you would use the full 3 varistor, (see article I referenced earlier), including the paths to Earth. The cabling for the SPD should ideally be at least 4mm2. Some commercial units that Iâve seen use very lightweight wire, it would carry very little fault current and would also have a high resistance, two features that would not help. If connecting between battery terminals or other low resistance/high current sources, then install a fuse in line to protect against faults in the SPD.
Ideally you would want the SPD activation voltage to be below the capacity of the attached electrical goods but this cannot always be achieved. For example, you could be using an MPPT with a maximum string voltage input of 150V, if your solar panels have a Voc of 100V, then you could select an SPD with 125V activation. This would be ideal. But of course, the existing string voltage could also have a Voc of 140V, so the SPD activation level have to be above the ideal 150V input level of the MPPT charger. Even in this condition, the SPD will provide a very high level of protection to the device from a spike because the MPPT has capacitors on the input which take time to charge up and exceed the rated 150V, (they might be good for 200V before failing), the SPD activates before the voltage inside the MPPT has time to rise and hence stops the bulk of the surge.
So where to place and what to protect are important but you need to consider the above before selection and installation.
BUT⌠If you are talking about preventing a spike from a direct hit by lightning, then sorry to say there is nothing that can be done cheaply to help there. An SPD is limited because it can only provide short term protection. Eg nano seconds. If someone connects a 230VAC line to the battery, the SPD will activate but it will soon fail too. They are only good for short duration spikes.
Hope this helps.
Ian
