Shore Power Cord Connector Missing Prong

[lawndoctor]

The male end plug on my Hubbell 50A 125/250V shore power cord only has two prongs. It plugs into a female receptacle at the dock that has slots for three prongs, that is, for two bent prongs and one straight prong. The missing prong on my end plug is the straight one, the two I have are the bent ones. Polarity at the outlets on the boat test as correct. Does anyone know if the third straight prong is supposed to be missing on the male end plug? Thanks for any input.

 

[ocgrant]

Not supposed to be missing. I would replace right away.

 

[ttmott]

It could be correct. You must have an isolation transformer on the boat. If that is the case then the shore power neutral conductor isn't needed nor used. The outer jacket on the 50 amp plug is the ground then the two prongs are the 120 volt hots.

 

[Bt Doctur]

All power cords are 3 wire, Hot, Neutral and Ground
https://catalina30.com/TechLib/Wiring Diagrams Boat/Marinco Boater's Guide to AC Electrical.pdf

 

[Bt Doctur]

It could be correct. You must have an isolation transformer on the boat. If that is the case then the shore power neutral conductor isn't needed nor used. The outer jacket on the 50 amp plug is the ground then the two prongs are the 120 volt hots.

Read up on electrical circuits,

 

[lawndoctor]

Read up on electrical circuits,

Thanks for the responses. I just read up on isolation transformers and polarization transformers, and I now have at least a basic understanding of their functions. If the boat has an isolation transformer, then the ground is not connected to shore, hence the ground prong is not used. I’m almost certain that is the setup I have, though the owners manual is not specific about it, so I will check with Princess.

 

[ttmott]

Thanks for the responses. I just read up on isolation transformers and polarization transformers, and I now have at least a basic understanding of their functions. If the boat has an isolation transformer, then the ground is not connected to shore, hence the ground prong is not used. I’m almost certain that is the setup I have, though the owners manual is not specific about it, so I will check with Princess.

Nope - the ground IS Connected. The neutral IS NOT Connected.
There are four conductors for the 50 amp 250 volt service -

1. 120 VAC Hot (phase A)
2. 120 VAC Hot (phase B)
3. Neutral (not used if boat has Isolation Transformer)
4. Ground (the outer shell of the plug assembly)

The internal iron core of the isolation transformer is isolated from the boat and is grounded to the shore side of the electrical so if there is a fault on the primary windings (the shore power side of the transformer) it is safely grounded.

 

[ttmott]

X = 120VAC Hot (Black Wire)
Y = 120VAC Hot (Red Wire)
W= Neutral (White wire)
G = Ground (outer shell of plug) (Green Wire)

 

[ttmott]

Here is a clip of a diagram for a 240 VAC Isolation Transformer and how it is wired -

 

[ttmott]

Read up on electrical circuits,

Alluding I'm incorrect?

 

[lawndoctor]

Now I’m getting confused again. I have read in multiple sources that isolation transformers sever the ground connection between shore and boat, that is, there is no connection between the shore ground and the boat ground. (E.g., https://www.passagemaker.com/technical/the-ins-and-outs-of-shorepower-transformers)

 

[ttmott]

Now I’m getting confused again. I have read in multiple sources that isolation transformers sever the ground connection between shore and boat, that is, there is no connection between the shore ground and the boat ground. (E.g., https://www.passagemaker.com/technical/the-ins-and-outs-of-shorepower-transformers)

You're correct, you do not want connection between the boat's grounding / bonding and the shore power ground IF there is an Isolation transformer for galvanic corrosion reasons. However, it is required to have the shore power ground internally bonded within the isolation transformer; The shore power grounded elements of the isolation transformer are isolated/insulated from the outer case of the transformer and consequently also isolated from the boat's grounding system. This is to ensure if there is a fault on the shore power side of the transformer the current safely routes to the marina's ground system.

Now, per code the isolation transformer is considered a power source like the boat's generator and as such it derives it's own hot, neutral, and ground therefore the ground and neutral must be bonded together which they are if you look at the robust bonding strap between the boat's neutral and grounding buss bars.

Lastly, don't confuse a 120 VAC isolation transformer with a 240VAC isolation transformer. The 120VAC transformer uses the shore power 120V Hot and Neutral as the power (like in a 30 AMP marina power connection) and generates a cooresponding Hot and neutral on the secondary side. Conversely, the 240VAC transformer uses two 120VAC Hot legs (out of phase) as the power and generates two 120VAC hot legs but different from the previous it has a center tap on the secondary windings that is the derived neutral which is used for the boat's 120 volt appliances. The marina's shore power neutral is not used.

Hope this helps a bit...

 

[lawndoctor]

Got it, thanks for the detailed explanation.