I think a granny charger (Poor Choice Of Name) is nothing more than a 16amp rated extention lead with a 13amp plug that connects into your car.  The higher rated plug and cable is because this may be drawing power for many hours or days to charge an EV. 

These leads all have a "blob" full of electronics in the middle that performs the same functions as a wall-mounted charger (EVSE).  They need to have something that tells the car how much current it can draw (usually set to 10A), something to disconnect the outputs until the unit has successfully  communicated with the car, and an RCD function for safety.

24Hour said:

When 2 vechiles are simultaneously accessible, ie in a street charging next to each other , will the open pen device ( assuming each charger has one ) operate if for example they are fed via 2 differing supply transformer earths ? .

It will if there's an open-PEN fault, but open-PEN is not the big issue here. In fact, it's an issue for any installation where ADS is used ... including TN-S and TT installations that are not affected by PME, as well as TN-C-S.

A more serious issue is that this arrangement is not conformant to BS 7671. BS 7671 is not guaranteed to protect against shock from simultaneously-accessible exposed-conductive-parts connected to different earthing systems, and is a fundamental requirement for Automatic Disconnection of Supply - see Regulation 411.3.1.1 (highlighted 2nd para):

411.3.1.1 Protective earthing
Exposed-conductive-parts shall be connected to a protective conductor under the specific conditions for each type of system earthing as specified in Regulations 411.4 to 411.6.

Simultaneously accessible exposed-conductive-parts shall be connected to the same earthing system individually, in groups or collectively.

Conductors for protective earthing shall comply with Chapter 54.

A circuit protective conductor shall be run to and terminated at each point in wiring and at each accessory except a lampholder having no exposed-conductive-parts and suspended from such a point.

The need for a Simultaneous Contact Assessment is discussed in Section 3.5 of the IET Code of Practice for EV Charging Equipment Installation.

Are Granny chargers generally not equipped with integrated open PEN protection devices? I thought these chargers are commonly utilized as interim solutions pending the installation of a permanent charger.

AMK said:

Are Granny chargers generally not equipped with integrated open PEN protection devices?

No, there are two flavours of "granny charger:

"Mode 1" which is, sadly, just an extension lead. These won't work with new vehicles, and it's no longer permitted to sell EVs that can use them.

"Mode 2" has an in-cable control box (ICCB) that incorporates some residual current protection, and N-E monitoring via the "pilot" function between the ICCB and vehicle.

The ICCB, being international standards, doesn't include an open-PEN device (OPDD) - so if you want to dedicate a socket-outlet to EV charging using Mode 2, you'd have to provide something in the electrical installation to conform to Reg 722.411.4.1