Air source Heat Pumps and Diverted neutral currents

Question

Hi I have been having a conversation around the requirements to fit an RCD on these circuits, it turns out it is a manufactures or appliance standard recommendation. It then occurred to me what out diverted neutral currents if the protective earth & neutral conductor (PEN) were to break. 
 If these units are fitted on a TN-C-S earthing system and the PEN conductor breaks the metal casing could become live as the earth conductor could end up carrying the neutral current. 
 The person touching the ground would be earthed and the current would flow through them, in this case an RCD would not function as it wouldn&rsquo;t see the imbalance. I feel it should be considered its no different to having an electric vehicle charger and having a metal car. For that an Open PEN detection device is required. 
 Any thoughts on this point ? 
 Regards 
 Daniel

AJJewsbury · Accepted Answer

I feel it should be considered its no different to having an electric vehicle charger and having a metal car. 
 
 As with anything, there are pros and cons. EV's aren't the only "Class I" item outdoors - think street lighting, domestic outside lights, external boilers, metal framed/clad buildings, those green boxes at the side of the road, and of course non electrical equipment bonded to the PME terminal - gas pipework, and outside taps. Unlike EVs there's no demand for open-PEN detection on any of those. 
 One of the big considerations is how likely is someone to touch the thing during an open-PEN event - an EV is very likely to be grabbed hold of several times a day (not least when connecting/disconnecting) - but when was the last time you actually touched a lamppost when you walked past one? 
 Add in that some of the means of protection can increase other risks - e.g. changing to TT increases the risk of earth faults remaining connected due to a faulty RCD - open PEN devices put a moving contact in the PE - so increasing the changes of a failure of basic earthing (and the common types don't necessarily spot all dangerous situations anyway). Some methods - e.g. introducing an insulating section in pipework or supplementary electrodes are generally lower risk and are often recommended, but have limited application. 
 So often it's less of a case of "identifying a risk and eliminating it" but rather balancing one risk against another. 
 - Andy.

AJJewsbury · Answer

If it's of any reassurance, I've just had a prod around my newly installed heat pump with a multi-meter. The main front grill is plastic, the rest of the (steel) case is heavily coated with enamel type paint and the pipework is covered with thick thermal insulation - the only places I could find continuity were a few small screw heads (none on the front, one on each side and a few at the back). Even the mounting feet seemed isolated. 
 - Andy.

Air source Heat Pumps and Diverted neutral currents

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